Swelling of the lips IS serious. Along with hives this would be considered an anaphylactic reaction requiring an epi pen (per allergist recommendations).
If the hives are only in the bum area, is it after a BM? If the baby is just starting solids, that could explain that. The acids in the foods and BMs irritate the skin.
Have you kept a detailed food and symptom log? That's really the best way to see what's up. Anything that caused lip swelling I would remove from your and the babies diet and speak to a board certified pediatric allergist who specializes in food allergies. Allergy testing is inexact at best (50% of positive results are false positives but a negative result is 90+% accurate).
Good luck. It's tough to figure it all out.
Yes it was very scary and without a doubt serious. I've since started carrying benadryl with us at all times, but the other day I was looking at the directions, just in case I needed to use it and noticed it should not be used unless the child is four years of age or older. My child is just 18 months. Is there a benadryl suitable for toddlers? A relative ran to the store upon the first reaction and I stayed with my child and the swelling went down rather quickly, so that's why I'm unsure of what's available. We obviously avoid dairy like the plague now, but I'm still concerned about the fruits that caused the hives. The hives didn't appear after a BM and lasted for a few days before becoming scally and eventually healing all together. My little one didn't seem phased by them at all though, which I found strange. I did assume it was the acid, considering the fruit exposure included peaches, raspberries, and strawberries.
Goodness, it is so much information to take in. Thanks for sharing what you know! :)