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.