Ive had that too and I think it is probably your own intestines. (Not very exciting I know). As the womb gets so big all the intestines are pushed out towards the sides more than normal and things pass more slowly through too. I had what I thought were labour type cramps but the doctor told me it was just bowel cramps cause the intestines just about surround the uterus by this stage. Unglamorous but good to know I guess.