How To Get Rid Of Welts?

I have a 3 year old with a low grade fever and lower body welts. He had a fever of 101 last night and we gave him some Tylenol. He still had a fever this AM of 99 but has been acting fine. We went to a children s museum this morning for a birthday party. He was fine the entire time we were there and he did not eat anything. We went to lunch about an hour after leaving the museum and when we sat down at the table, he started complaining that his foot was itchy. I observed some small welts on both feel similar to what you would see after a bug bite. Within about ten minutes, the small welts had turned into large welts and traveled up both legs and to his lower back on both sides. My husband left the restaurant to get some benadryl at the walgreens next door. We gave him some benadryl and it seemed to help in about a hour. I know from my other son that the welts can remain for some time, but now I m noticing some welts starting on his cheek. They are very small, but I m a little concerned. We just gave him some more benadryl and more Tylenol since his fever has returned. While at the museum, the only place we went that might have caused some reaction in him was the butterfly garden, but he didn t touch anything or walk in any grass. Im trying to figure out if he is just having a reaction to something he encountered there (environmental) or if it is somehow related to his known food allergies (He has a class 6 peanut/treenut allergy on the rast scale and a class 3 for egg). Again he didn t eat anything, but we are starting to get concerned that it could be something like a touch reaction. He hasn t been in a situation where he has encountered peanuts or treenuts before and honestly we are not sure how sensitive he is to touch. He older brother also have a severe peanut allergy so we had the younger one tested to see if he also had the allergy. Since we found out he does, we ve kept him away from exposure just like his older brother. I did notice a peanut shell on the floor at the restaurant we went to today but it was outside and we just walked by it. He didn t touch it at all. I m not sure what to think about it or if we should be concerned now that is has been hours and he still seems to have some welts periodically forming.