Low grade fever

My little hasn’t been feeling good all week. She’s been sweaty, irritable, and just seemed unwell. For the sweatiness I would take her baths every night to regulate her body temperature which seemed to work and it helped to comfort her. I wanted to see if it would pass as I didn’t want to take her to the ER and expose her to any worse sickness. It is now Friday and earlier she had a rectal temp of 100.0 but went down within about 15 mins after I had undressed her to take a bath ( I wanted to check twice with two thermometers just to be sure). It went down to 98.6. What should I do? She’s only 3 months old.