I had a partial knee replacement on 3/10. All has been going well, but yesterday I started to develop a rash behind my left knee. The rash resembles heat rash and is very itchy, so I took Benadryl. The rash, however, is spreading up my thigh, onto my belly and buttocks. I do not have any swelling in my throat or tongue. I am unable to reach my doctor. How can I treat this?