Horrible rash

My 3 year old suddenly got this red rash with a bunch of bumps behind both of her ears and the back of her neck around her hair line. She said it hurt and itched but it couldn’t have been too bad because she was up and playing like normal. We haven’t changed any shampoos, soaps, or left the house today so she wasn’t around anything new. I’ll be taking her to the doctor tomorrow but in the meantime I’m still stressing. If your little ones has had anything similar please let me know what it was.