How old is she? If she's 13 or thereabouts, then she may be experiencing bowel changes along with hormonal fluctuations. I know I used to get constipated before my period. If your GP doesn't have any answers for her, then try taking her to a gyno and getting her hormones worked-up. Even if she comes back normal, she may still want to try BC, as sometimes that can make bowel problems better (sometimes worse--depends on the person and the BC--nothing to do but use yourself as a guniea pig). She may also outgrow the problem in a few years as her hormones settle down a bit.
With Crohn's in the family, a colonoscopy might not be a bad idea. It doesn't tend to show up, as far as I know, until the late teens into the 20's, but there has been one lady on here with a child with Crohn's, so it's not unheard of either.
Have you asked her about the nature of her stools? Are they ever yellow (or the toilet paper yellow), or do they burn? These can indicate a gall bladder problem, which also causes sudden fullness and then sudden hunger. Of course, even having regular IBS can cause this too, as you can start to eat, feel your intestines reacting and stop eating--no longer feeling hungry; then, when the guts calm down, you're starving because you never ate a proper meal. But IBS doesn't produce yellow, burning stools nearly as much as a gall bladder will. If they are ever black or she has blood on the toilet paper, make sure you tell this to her doctor as this can be a sign--as I'm sure you know--of Crohn's or possibly of a stomach ulcer.
I know what she means by "feels weird"; I've never been able to properly describe my symptoms when my guts are acting up. It's half actual feeling, half intutition, I think. I may not feel sick right this minute, but something is telling me that if I eat, I'll have D. Or if I don't take medicine now, I'll have D shortly. Like I said, there aren't really adequate words to describe it--unless you pun and call it a "gut feeling". That's really what it is.