C.K.
B.,
She could be lactose intolerant. Try taking her off of all dairy for a week. Then reintroduce it. If she is still not tolerating it, it is likely lactose intolerance rather than an allergy.
Speak to her doctor - there are products on the market for kids (i.e. Lactaid), but I don't know the age at which they can start such a product.
C.