Hi Esther!

Your mom is right. You can suddenly develop an allergy to a food that you have eaten for years without a problem. My DH had that experience last year and wound up having allergy tests because of it. He started having his throat swell whenever he ate raw apples, peaches, carrots or celery and he'd eaten them all his life. Unless you have an allergy to all dairy, it's probably not the buttermilk. Buttermilk is just milk that has been soured by adding a bacteria culture or vinegar. Hmmm...there's that vinegar again.
Unless you're willing to get tests for allergies, the only way to find out what you're reacting to is by trial and error and since the reaction tends to be stronger each time you're exposed to an allergen, that can be risky. You might want to contact your doctor or an allergist before experimenting on your own or at least have some Benadryl handy. If you happen to experience any tight feeling in your throat or chest or have difficulty breathing along with the hives and itching, get to an ER right away.