http://en.wikipedia.org/wiki/Intussusception_%28medical_disorder%29 is a description of intussuseption. Please read it and if it looks familiar, talk to your doctor again right away, or get a new doctor.
There have been lots of sickness like this in this year if you look at the other posts here,with similar.it could well be some virus we have had. I think as they have been neg with tests they took, give it longer, make sure she is hydrated , and try soups, puddings, ask her what she would like to eat .If it continues take her back for further investigation .good luck