What Causes Body Tremors In An Infant While Suffering From Fever?

Hi there, I have a 13 mth old and she suddenly got sick. She was perfectly fine all day and at dinner time but around 7:30 at bath time, her face suddenly lost color and she was shivering in the tub. I took her out and got her warmed up and ready for bed and kind of just put it off thinking it s just getting out of the bath. I checked her temp and it was around 100.4. As she was drinking her bottle, I kept checking her temp because she felt that she was getting warmer. Her temp was all over the place. It had checked as high as 103.3 then I d wait a few minutes and check again, and it d be 101.8. It seemed to be going up and down very quickly. So I immediately put her to bed to rest. She fell asleep really quickly by 8:30 and I watched her sleep for a little while. She was twitching all over her body so I just figured she was extremely in a deep sleep in REM sleep. She just woke up 3 hrs later and is radiating heat but her temp was only 101.8 but she was slightly shaking and seemed very uncomfortable. So I gave her baby Tylenol and placed a Kool fever gel pack on her forehead. I m holding her now but her eyes are open and she s twitching all over her body. Am I just being paranoid or is there something I need to watch out for? Any thoughts help. I m hoping the Tylenol will just kick in and it ll bring it down. The shivering and the shaking is just something I ve never seen as a symptom when she s been sick with a fever before.