So, turns out my car might have blown a head gasket. Fun times. It's overflowing way too much coolant into the overflow tank, so possibly exhaust gases getting into the system.
I know nothing about cars, but I do know I had a friend blow a head gasket on his Subaru Legacy wagon, and it was gonna cost more in labour to fix than the car was worth. Hopefully your car makes it easier or you can do it yourself!
Definitely not, it's a late model diesel, mazda CX-5. I'd never get it back together if I did that.
It's also been doing the coolant thing for a while, so it's not getting any worse. I might just nurse it along for a while, you just need to remove the radiator cap and blow into the overflow pipe to push it back into the radiator.