This pretty much describes my daughter. She ate a cashew when she was 2 (surprised the hell out of us, because she never eats ANYTHING) and broke out in a rash. Turned out she was allergic to cashews and brazil nuts, but they warned us away from all tree nuts. We kept an epipen and Benadryl on hand, but since she's a picky eater it was never an issue. The most she ever had was an itchy throat, easily treatable with a Benadryl. She has since outgrown the allergy, but we still treat them with caution.
My son has a friend with a peanut allergy. He will break out in a rash if someone who ate peanut butter even touches him. They have an epipen in the office at school, and when he's out of school, he wears it in a fanny pack that is constantly with him. His mom has to be vigilant, but he is fine.
Good luck, I hope it is nothing serious!