Advice Please?!

My 2 year old has a fever. She was fine all day yesterday but had a fever last night of about 101. She was fine again all day today and then began the night with a 103.5 fever that we worked down to 101.2 with medication. Shes REFUSING to drink anything. She keeps asking for milk, which I give her and she takes one sip and cries that she wants milk. I offered juice. I switched the lids on the cup, offered a different cup. Even offered a bottle (which shes never even used before because she was breastfed). I called the on-call doctor and she said "if the fever breaks 104 or shes very sickly then take her to the ER"... but I do not know what to classify as very sickly.

I should add she drank a TON of drinks at my moms today. We got home around 430 (11pm now)and shes refused everything since (thats also when the fever struck).

I already gave her medicine y'all. Why isn't anyone actually reading. I am not asking how to keep the fever down. I am asking if anyone knows what can cause a fever to come and go.

I already called the on call doc. said that too.