Unfortunately it sound like your mollies have an internal parasite. Is there any red thread like worms hanging out the swollen part?
Here's a post about my experience with the red worms. See if the symptoms are similar. I hope not, because this is a terrible thing to go thru. I have pix of my angelfish (last survivors),
betta and keyhole cichlid who were all infected and died. The are pix of the anus and the worms. We lost a lot of fish.
I truly hope it not the same thing.
My experience with Camallanus worms