R.P.
Is it that she just does not have any poop or she is trying but just cannot go? If she is trying, she may just need some help. Our ped told us that babies do not always "know" how to use all of their body parts (especially that one). She may just need some stimulation to help her learn how to appropriately use that particular body part. Please consult your ped first, but ours told us we could put petroleum jelly on a cotton swab, gently rub around & just inside the rectum. If it comes out "dirty", the potty is definitely there, she just may not know how to relax those muscles when she is pushing. You can help her by massaging that area with a lubricated pinky finger & doing bicycle motions with her legs. Our DD1 had this problem & did not fully know how to potty until she was about 14 mos old. She's right as rain now & we know how to help her sister should she need it.
Again though- I'm not a medical professional so please consult your ped first.
Good luck and hope this helps!! :-)