well bloat is usually because of overfeeding. The tank is obviously filtered and clean, so you probably just overfed or something. To treat bloat, take some thawed, deshelled pea. Mash it up and feed to the sick fish. Also, reduce the amount of food you are feeding or prevent her from eating so much. Usually bloated fish will be very sluggish, and stick to the bottom, or can be found seemingly lifeless on the top of the tank. If your fish is still active, and if she is in fact a molly, then she is probably pregnant. Is this fish a male or female?
Most likely, she is just pregnant. Be prepared for lots of fry! Mollies are very prolific breeders so....