When you pull the dipstick, what does the oil look like? Chocolate milk?
If you’ve got water in the oil, you’re in for a pricey fix, though maybe not 2-3K. It’s not out of the question at all @ 125K that you have a blown head gasket.
You need to fill the radiator because (as has been explained) gases from the engine are getting into the coolant system, which’ll cause bubbles.