I have this exact same problem. It started about 4 years ago. I am 56 years old and have worked under cars all my life as a hobby, not professionally. Now, when I begin to crawl under a car on my back, the “heavy headedness” and dizziness come on almost immediately. Within about 15 to 30 minutes, I cannot stand the nauseated feeling that comes on. If I sit or stand back up, it stops but I am still nauseated for hours. Two days ago after working under my car, it was so severe that I had to go lay down in bed. What you say, lay down? I thought that was what caused the nausea? No, laying down in bed, on a couch, or otherwise getting horizontal does not cause the nausea. I have talked with my doctor about this and we cannot figure out what causes this nausea. I do take Prilosec for heartburn and indigestion but I had been taking this via doctor’s prescription for years before this all started. So, OP you are not alone.
Claustrophobia is my guess. Lying face up in a confined area where you need to reverse the creeper with unnatural arm and leg movements to escape.
Same thing would happen to me if I tried to do a repair in that fashion.
I’ve never experienced that symptom as a result of working on cars, either from above or below. However it definitely seems possible the fumes that exist on the underside of a well-used vehicle could trigger nausea, allergic, even psychosomatic symptoms. Fumes from gasoline, oil and sludge, power steering fluid, coolant, gear oil, transmission fluid, windshield washer fluid, any of them could be problematic, especially if you were already sensitized. Similar to how sometimes a person will eat something that disagrees with them, makes them ill, and afterward, months or years later, they experience the aroma of that same food and again they immediately feel ill.
The only health problems I’ve experienced working on cars – besides one case of dizziness caused by accidental CO exposure – is allergic skin reactions to solvents. After I figured out what was causing it I started wearing latex or nitrile gloves when working w/solvents and that solved the skin-allergy problem.
I could certainly understand the grease and fumes. I think I would try to recreate the situation. In the article the dude said he didn’t have problems while not under the car. Sure, he’s laying on a chase lounge, not lying on hard concrete, holding hands above him to exhaustion, with neck, head and spine contorted.
As a test, maybe try buying a comfortable creeper, slide under the car and lie there 30 minutes (maybe with eyes closed) and arms resting calmly. Maybe play some music.
Vertigo is sneaky stuff. Little granules in the inner ear come loose, sometimes when the head is in an odd position… often when lying down.
Personally, I get similar, but much milder, to your symptoms. It could be less oxygen than I had 10 years ago. Or get severe pains from lying on my old creeper or cement, so have little arm strength. Mostly it’s pain in back, neck, and muscles, but occasionally it’s been in my wallet when I realize I’m not going to fix this part.
I wonder if it could be nausea from a vision problem due to the natural aging process? I remember in my late 40s being disoriented the first time I stuck my head in a wheel well and could not see the brake pads because they were now too close for me to focus on. At one time I tried a pair of blended bifocals and could not adjust to them, they made me dizzy.
Or suspend a transmission over your bed 
Thanks for reviving this. It led me to think of a new guess as to what’s going on – and I’m going to see if your situation supports it.
Do you wear glasses? Or maybe started wearing glasses around the time this started?
A rough guess as to when this started for me is when I started wearing progressive bifocals.
My new guess is that the need to focus on the short distances (the underside of the engine area) while under the car contributes to the nausea. That doesn’t happen when I lie down anywhere else. And if I try this with the covers while in bed, I don’t have my glasses on and so the situation is different.
I don’t get nausea but I am very claustrophobic so in a tight situation I do get into a bit of a panic mode.
Many years ago I went for a closed MRI on my back. The minute the gurney hit the tunnel there was a white flash and I was out cold.
Yeah, that’s the spirit. With me, I figure it was a combo of high outside temps, the creeper, and vision. Parts are always somewhere between glasses and normal vision.
Melissa , do you even pay attention to these threads when you post .
Hear that!
… similar to what George_San_Jose1 said previously.
Could be an early warning sign of a heart problem - especially if you’re in your 50’s - which is the time in life when all the years of eating the wrong foods starts to catch up with you, clogging your arteries.
Do you experience some light-headedness when you stand up? Have you noticed a restless heartbeat that may be affecting your sleeping at night? Have you had any trouble getting on top of your breath at times? How about indigestion? Do you feel more tired than you used to? Are you diabetic (even only type 2)?
Heart problems do not necessarily cause any chest pain. If this feeling persists, you might want to get an exhaustive heart test - like a stress test. You may have a blood flow problem.
This topic caused me to spend a lot of time thinking about wearing goggles while working on vehicles!
Funny you mention that. Sound proofing one of the doors today, I started getting dizzy, nauseated. I slipped on a pair of dark safety goggles and it went away. At first I thought it was the material.
no to all your questions – this year’s physical showed no problems
I get very bad nausea any time I lay on my back and try to work on the underside of a motorcycle or vehicle whether it’s on a creeper or not. So bad, I’m still laying in bed typing this right now with the AC blasting on me. I think it’s from trying to move my head all over the place and focus on very close objects… I do wear eyeglasses. Never get it when I’m just relaxing on a creeper though. Just moving my head and working on stuff upside down. I also get severe car sickness if I try looking down while going through the city.
I have had this very problem for several years. I’ve spoke with my doctor and a cardiologist. Nothing.
If I lay on my back and work with my hands over head I will become sicker then a dog until I stand up.
Then it takes about 15 minutes to get back. I sleep on my back with no issues.
Are you working with your knees raised? Are you using a creeper with a cushion to raise your head?
I am unable to lie down with my knees raised when working out unless I very gradually get down and even then there is some dizziness occasionally. My problem seems to be age related as it has grown worse in recent years. When working under a vehicle I put a cushion over the one on the creeper which helps some but getting up can be difficult.
And hopefully Miss Carolyn will reopen this as a new thread…
I discovered what it is. My doctor had suggested it but I didn’t buy into it until I stumbled onto description of BPPV. benign paroxysmal positional vertigo. It happens when your head is in the right position and you move it from side to side. I don’t feel ill just laying on my back. But when I use my arms. But when your work with your hands you shift your head from side to side which causes the Vertigo. I can even bench press with no problem. https://aaptiv.com/magazine/vertigo