I'm with everyone else. Usually when my eldest has stomach problems, it means there's something going on that she's having trouble putting a voice to and is looking for attention (subconsciously in my daughter's case.) However, it wouldn't hurt to take her to the doctor, and if nothing pans out there, maybe consider a counselor.
My daughter loved her counselor and it gave her a venue to really air out her feelings (though it took a while to get to them, she tends to emotionally "stuff") in a safe environment that's not mom (sorry mom, no offense.) I worked with the counselor as well so that by the end of our sessions, my daughter was able to tell me what was going on, knowing I wouldn't judge her and I would help her through whatever it was (which so far hasn't been big stuff, but her teens are coming up!)
Do your best to really be there for her and / or connect her with a trusted adult (female preferably IMHO) in your stead who she can talk to openly.