lower abdominal pain

HI there...
I just posted this in another thread, then realised there was a twin thread to use!! Sorry for the repitition...

Bottom line is this. I am having bizzarre lower abdominal pain (just above the pubic bone), that comes and goes and at times is very uncomfortable. Then I have it around the back as well, like maybe my uterus is pushing down on my intestines and trapping air or something (this is REALLY hard to describe without grossing peopel out...) It makes me feel like I should go to the toilet to relieve the pressure (like gas or something), but then it's gone and I'm fine again.

This is my second pregnancy, and obviiously very diff to my first (she was a singleton). Just wondering if anyone else experiences this? Is it just round ligaments, or my uterus stretching, or maybe something worse like pelvic inflammatory disease? I am seeing my OB friday, but wondered if anyone else had any input/advice for me...

Julesy