Brief Answer:
Most likely some febrile viral infection

Detailed Answer:
Hi

Thanks for asking on this platform

I have gone through the query and do understand your concern.
Fever in this age is usually due to some viral infection. Bacterial infection is less common but should be kept in mind if fever persists longer.
In viral infection fever usually persists for 3-4 days. If it persists longer blood test and urine test should be done for bacterial infection.
I would like to know about pattern of fever. Was it of high grade initially and is showing declining trend now? Is there any associated symptoms like urinary difficulty, cough, cold, sore throat etc.
Child may become lethargic due to fever itself. I would suggest you to give Tylenol for fever. Give her something to eat as poor oral intake may lead to low blood sugar and thus poor activity.
If she remains inactive even after the fever subsides or if fever persists longer please take her to a pediatrician.

I hope this helps.
Please let me know if you have any more doubts.
Regards

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