Reason for hives on kneecaps after a minor reactions?

Description: 17, male, 120lbs, white.

I am very sensitive and have allergies to most common foods (egg, milk, fish, peanut), so I am frequently having reactions. Doctor said it can't be diagnosed because there's such a range of possible sensitivities. After keeping track I noticed that every time I have a minor reaction my knee caps are the first location where spotted hives pop up. Even hours after the reaction subsided (by taking a benadryl pill) the locations are still red and slightly raised. Is there a reason for this? Does it mean anything regarding my sensitivities?