I don't want to scare you, but we just went through this. My daughter had an unexplained fever for 10 days. We went to the ped 3 times and had many tests done. Everything from nasal cavity swabs, to strep test, to chest x-ray, to blood drawn...they found nothing! They finally admitted her to the hospital when her temp reached 105.1 (ear thermometer)-we gave her tylenol and 40 min later when she was in triage it was only at 103.8. They observed her for two days, gave her fluids, antibiotic, and repeated many tests. Still found NOTHING!
Keep taking her back to the doctor and ask them to keep looking for the reason. I didn't settle for the "its just a virus" because the fever kept coming back. They found her white blood cell count was high and we have to keep watching her. Any signs of fever, vomit, diarrhea (sp?) we have to take her back immediately to the hospital. They will then refer us to Infectious Diseases at Luthern General.
A fever is just a symptom of something else. It's her body trying to fight off some type of infection. Keep her comfortable and don't let it go. I called my doc just about every other day, or when I noticed a new symptom. I know you feel like you're bothering them, but it's their JOB! Your baby is the most important thing, not the doctors opinion of how much you have bothered them.
I hope your experience doesn't turn out like ours. I will keep you in my prayers.