Your fish has gas...its a disorder and you can look it up and get the proper name! It is very common and can kill your weak goldfish
Why Is My Goldfish Floating On It's Back? The Fish Is Still Breathing And Will Continue Swimming After Floating For About 20 Seconds Or So. The Fish Is Still Eating And Has No Visible Irritations.
It sounds like a...can't remember the proper term...flotation disorder? It actually is a fairly common problem with goldfish where they lost their ability to balance in the water. I'm not sure how expensive it is, but they can inject cork in her back to keep her upright in a more comfortable position. Yes, there are actually fish vets.
My fish is to but I got it at wall mart and it waz not doing it thar.