Hi there,
I agree with the other ladies' advice. I would not expose LO to the ER for this right now, especially if benadryl is helping. As far as anaphalaxia, that's typically SO severe you see it immediately,,,,,,wheezing, swelling, respiratory distress, etc. As Heidi mentioned. Not that you shouldn't be concerned and keep monitoring, but it would be pretty obvious. I would make an appointment with your regular ped, keep a record, and take some pictures.
My older daughter, who has no food allergies (but is very allergic to penicillin) used to get hives like that periodically. It was so strange! She would get them on her legs, arms, etc. We could never pinpoint the culprit. First time was when she was about 18 months, and we were at the beach......it was either sand flies, or Mosquitos. Always went away and didn't seem to bother her. But freaked us out a bit!
How is LO doing now?