With the heat starting, I'd assume first that it's heat rash. My son gets it on his back, but I would think that behind teh knees is a very sweaty place. Is she still wearing heavy pants or bottoms? Try switching to something lighter and maybe no lotion in the morning? Or light sunscreen and sandals with no socks....
If it's spreading and itchy and it's not heat rash or the obvious chicken pox, another thing to look into might be scabbys which generally start in sweaty areas, in between toes, back of knees, inbetween fingers. It's VERY common, even though we all think it's gross, but it's a communicable so they can get it at parks, pre-school, daycare, etc. It's an easy fix to get rid of, with a lotion that you put on twice a day I think for a week or something to that effect.
Good Luck, hope I didnt scare you, but it doesnt' sound like Eczema to me these are just things to look into.
K.