That's quite a quick reaction. It could be an allergy or it could be a contact allergy or it could be an intolerance (not much help, am I). It has been recommended to us that we challenge Minky every couple of months with milk to see if she starts to tolerate it any better. We aim to give it to her for two weeks and see how her skin is over that time. With yogurt, it was fine. With milk, I called it off after a week as it was clearly having an effect (skin drier than usual, which is a step closer to a nasty outbreak for her). You might want to give it to her for three days and see if the reaction is the same rather than there being an alternative cause. And then possibly leave it a month? I started Minky on dairy at 10 months as I was beginning to wonder what we would do when I stopped breastfeeding. You might want to start thinking about alternatives to milk, if you do think there is a intolerance, unless of course you want to continue breastfeeding, then there is no need to worry (in my opinion - normal muppet disclaimers apply).