My 15 Year Old Daughter Has Been Running A Low

My 15 year old daughter has been running a low grade fever(between 99.7 - 101) for almost 2 weeks. She had a sore throat at first and sinus congestion but all that has cleared up and she still has a fever. She has complained about her feet hurting especially her toes but no other problems. She just finished taking the z pac antibiotics and they haven’t helped at all. What could be causing the fever? She had a chest x ray and blood work and every thing was fine.