Why would it be that my stomach is always hurting after i have dairy when it never use to before?

Lactose intolerance: Milk products contain lactose which requires lactase (an enzyme) to digest, many individuals develop intolerance gradually in their early to middle years (espec, certain races). Lactase supplements may provide some relief, but alternative products or complete avoidance may be necessary. Calcium supplements, yogurt (less bothersome usually) would be a good idea to avoid bone problems later.