How long can a low grade fever last before I should worry in my 5 month old?

My daughter has had a low grade fever of 99.8-101 rectal for the last month. It always raises in the afternoon. I have taken her to the doctor and she thinks it is teething because she has tested her blood and urine and everything comes out normal. I am gettting worried because summer is here and I dont want her temputure to get higher.