My daughter 11 year old has fever 100.8 for 3 nights. no other symptoms, just a bit tired at times. no fever during the days, only evenings-nights. covid test negative. what could be causing it?

Viral illness usually the cause of fever in the evening and at night. The home Covid test has low sensitivity, some brands have only 50-60 % sensitivity which means the negative result could be wrong up to 40-50% of cases. I would not rule out covid-19 and will isolate her to high risk people to at least 24 hours after she becomes afebrile. You only need to watch her, make sure she's hydrated.

The timing of the home test is important to consider. When was the test obtained I. relation to onset of symptoms? A PCR test is more accurate. At day 4 to 5 of fever, additional testing may be considered. Other infections including Strep throat, Mononucleosis should be evaluated. Watch for signs of red eyes, rash and swelling.