Both my daughters at the same age developed an allergy to eggs. Around the age of 2 both of them started with rashes and constipation when they had anything with eggs but both grew out of it at the age of 4/5

You can become allergic to something. I've known someone who developed a nut allergy despite being able to eat nuts previously with no issue. I developed an allergy to my dog. Although I wouldn't say that it's specifically a dairy related allergy unless it happens with everything dairy related. Could well be a lactose intolerance? I am sensitive to dairy, gives me the 💩💩 tmi sorry.