Is this a new thing or has it been since you bought her?
My female mollie ended up with hers protuding constantly after about her 6th batch of fry (with me, I bought her adult). I reckon it's just like humans and childbirth Sometimes when things are stretched they just don't go back right!
Pics are best.
Maybe check out her gills? Compare the movement to other mollies, make sure her gills have a nice colour.