I know what you're going through with your fish. It's awful. I'm not familiar with Pimafix, but if it says it's a medication for internal bacteria, I would think it should work. I got this answer on another forum saying that with livebearers (I'm not familiar with yellow lab) these kind of things are seen often. The person thought it's most likely viral and not really treatable. Guppies often get the sunken stomach problem, but mollies can be prone to it as well.
That was a reply from someone else, but it's one possibility as well. I hate illnesses in fish that you can't tell clearly what it is and treat it knowing you're treating the right thing.
The main thing is to try to help with our best abilities. Hope your fishy ends up well.