I would ask the dr for an xray of her abdomen...or the next time it occurs take her to the er and ask for an xray.....what color is her vomit? how are her bowel movements? there are a few things that could cause her this problem such as an intermittent volvulus, or malrotation, or some other intestinal problem...the other thing is a gi specialist....the fever makes me think of these things. good luck
It could also be a simple hernia. A lot of people have hernias around their belly button from the cord, but sometimes, that can be really painful.
There is a rare autoimmune disorder called Addison's Disease. It only affects 4 in every 100,000 people, but it would be worth it probably to have her tested for it. It is also known as adrenal insufficiency, and it happens when the message from the brain to the adrenal glands doesn't get received in the right way and the adrenal glands fail to produce the needed chemicals to react to stress (or possibly exertion). During what is called an "Addisonian Crisis," immediately after the message has failed to be responded to, it can cause severe vomiting and abdominal pain, as well as other stuff the more it develops. Like I said, it's very very rare, but it might be worth testing for in this situation. (For the record, it is easily manageable.)
I also had a friend once with the same problem and it turned out that some of her organs were in the wrong place for some reason.