Are you certain that she was pregnant to begin with? Did you see a doctor? If she was infact pregnant and had a complete miscarriage, then y'all should have been protected by the condom, provided it was used correctly. I certainly hope that all of this was comfirmed by a doctor, and that y'all were given the go ahead for sex. The stomach problems and breast tenderness can be a side effect of her birth control. When I was on the pill my stomach constantly felt bloated and my boobs would hurt pretty often. If I were you, just to be on the safe side, I would call the doctor, or talk to him at the follow-up exam.

Most people's stomachs get bigger when they eat, it has nothing to do with being pregnant. Her nipples could be darker because of the BC, which is what it sounds like because they are tender as well, and a change in hormones (which is what the pill is) does that.

It doesn't sound at all like she is pregnant. If you are truly worried, a HPT should ease your mind.

if ur gf is pregnant none of that stoff will hurt the baby in the first three months of pregnancy. coz the body knows the mom may not know abt it and protects the baby.
but I agree with the others, it dosen't sound like ur gf is pregnant. take an home pregnancy test.