my tropical mollie won’t move her back fin when she swims and her stomach is really big and i what to help her

she has been staying in the same place for 5-15 minutes then she moves a little and she has a cut on her tail. also one of my other fish has been bugging her and then when he stopped her stomach got alot bigger. She is a tropical mollie from wal-mart and we got her with 6 other fish and 1 died in a few minutes of being in the tank, but a week later we got a new tank and 6 new fish. Then 2 more died and i got another fish of one of the same kind at petsmart and he has been bothering her.