Ummm, well...I had the same but it was about 17 years ago. What I was told was that a certain percentage of people who get Mononucleosis get a rash BUT since I had family members who were allergic to penicillin that I too might just be allergic to penicillin ( as a result I’ve never taken it again) The rash, unfortunately lasted about a month to six weeks, it was awful, I feel for you.