Hives!?!

Our whole family recently had some sort of virus (not Covid). It’s been a little over a week and we are still recovering a bit. This morning, my almost 4 year old woke with a huge rash on his bottom (by his underwear line). I put cortisone cream on and it went away for a bit. Tonight it flared up again, mind you I took his temp and he is still running slightly hot (99.5-100.9). I tried to get him into urgent care but they closed right before I was able to get there. I will obviously take him to the ER tonight if it gets worse or to the doctor tomorrow, but has anyone experienced this!?!