Suggest Treatment For Fever And Fatigue In An Elderly Person

My Mom ... she is 76 years old ... approx 100 LBS 5 ft 2 ... been in good health both mind and body , no pre existing conditions of concern. See her Doc s regularly and gets her Flu and shots every year. Normally eats and sleeps well, plenty of energy for her age, no stress. Lives alone with her two dogs. Earlier this week, she started feeling very tired during the day, and running a slight fever - up to 101. Checked her in the Hospital ... ran a number of tests ... blood, urine, cultures, head scan ... all came beck neg for infections. Cultures results are pending. White blood cell count went from initially high to now low within 24 hrs. Until they confirm something ... they are treating the fever with tylenol and IV fluids. No Antibiotics are being used. One event that happened last week ... she had a small tick (in southern NJ) that was removed within 6hrs that could be attributed to the fever and fatigue. Doc is running test for Lymes. If neg on Lymes ... what else could cause swing in white blood count, fever and fatigue? What other tests and treatment should be considered.