are her scales protruding in a pine cone type formation at all? if so, i would say she has dropsy, a highly fatal and contagious condition. euthanization is best if it is dropsy, and a complete disinfection of the tank and accessories will be needed.
could you post a photo or video of her? that would help us a lot. also, what did you last feed the tank, and when? are there male guppies in there too? what are your water parameters (ammonia, nitrite, nitrate, and ph)?