If your LO had a sensitivity to cows milk. How soon after introduction did you notice a reaction?

I suspect DD2 might be lactose intolerant because the few times I've had straight dairy she was really fussy and spit up a lot in the day following. At that point she was not a spitter any more.
Dad is lactose intolerant and so is big sister and my brother so there's a good chance! I had some milk on Sunday and Monday. And so far she doesn't seem particularly fussy. But she did spit up a ton this morning but not sure if it's from overeating?