Pictures would help, if you have any or could take a few of her. Is she passing her poop normally? Could be she's blocked up and needs to take a nice healthy dump, especially if she's passing gas. It also could be a blockage of some sort or something entirely different. Again, pics would help but if you think it's dire or an emergency, take her to the vet and forgo asking opinions on what everyone thinks it could be and go straight to a professional with her to hopefully find out what it is. At best I/anyone else could only offer educated guesses or opinions. Good luck with her and hope it turns out okay.
Is she gravid? Did you buy her as being gravid or did you pair her at all with a male? How old is she and what's her current or most recent weight? More information the better but again, in the end it's only going to be educated guesses or blind shots in the dark; if you think it's serious, I'd bite the bullet and take her to the vet.