Hi. My Daughter Is Complaining That Her Knees Are Hurting.

Hi. My daughter is complaining that her knees are hurting. She has on the inside of her knees circular red patches that she says are a bit like a wasp sting. It was sudden. 10 minutes before we were happily playing, althought she seemed to have restless legs. I even asked her if she needed to go to the bathroom. She also has been swimming today, and Probably didn't shower well enough.
A cold wet cloth seems to make it feel better. What is it?