My 3 year old has had a fever for 9 days low grade to 103. Her blood work was ok with elevated monocytes. .. ne for flu. No meds. What could it b?

Fever + ?? symptoms. Normal blood work except elevated monos suggests a viral infection, and there are other viruses in additon to influenza that can cause fevers, but usually there are other symptoms and findings on exam to support dx, ie, cough, congestion, diarrhea. Children this age do get infectious mono, but usually with sore throat and nodes. Has urine been checked for UTI? Repeat exam in 5 days if still T.
Follow-up. It is important to follow-up with pediatrician and follow the symptoms. It is still probably a viral illness, but the longer the fever goes on the more the medical team needs to think about bacterial infection, or other possibilities including autoiimune disease like rheumatoid arthritis or mediterranean fever, or possibly oncological reason.