Sudden dairy allergy

Up until last week my 4 year old had no known allergies. She would drink a glass of milk a few times a day , eat cheese & ice cream occasionally. Last week she got violently ill along with extreme constipation & vomiting, doctors didn’t really know what was going on & sent us home for basically me to figure out what was causing it. I only realized it was a dairy allergy after a few days of her only drinking juice & water. She got better but after I had given her a glass of milk she got sick again hence when I discovered it was the milk causing her sickness.

Has anyone else’s child suddenly had a dairy allergy? I’m lactose intolerant but only was I lactose intolerant my early 20’s never before that & it was more gradual. It seemed so sudden with my daughter it’s crazy.