Doesn't look like a typical cellulitis or erysipelas case. The knee doesn't seem to be involved, so I don't think it's some kind of arthritis. May well be prickly heat (heat rash).
@dofus kamas it's not good and polite to copy J.P.C. Peper's diagnosis,if you don't know or you got no idea you shall not copy other people's ideas.Don't get this personal.