I had a child years ago develop hives shortly after our lunch. I didn't send her home as no other symptoms, I just let mom know what was eaten. I'm embarrassed to admit, I had served white cheddar that I didn't realize in my rushing, had a sprinkle of white mold on it and caused child to hive. I only realized it in evening when I took out the cheese for dinner and noticed it then. I admitted to mother who didn't mind. No hives again after that. But if they persist or become huge welts which also happened to a child I had (who ended up having a nut allergy), I'd ask for allergy testing.