my 9 year old daughter has been running a low fever 99/100, off and on for 3 weeks. she s tired more then shes not. no other symtoms except mentioned pain in stomache once and a headache once, in this last 3 weeks. took her to the doctors, he said lets wait it out, may be a virus, but to call if the fever gets 101 or more. should I be concerned and what could this fever be from? thank you