Might a person be lactose intolerant only sometimes?

Yes: Depending on how much milk you have in your diet. More milk, more lactose, more intolerant. If you spread it out over a day it might not be as bad as if you have a large portion all at once.

The tipping point: Lactose intolerance is a fact & related to an absence of the enzyme to digest the lactose in your gut. Symptoms will depend on the products you ingest, their lactose content and the volume you stress your system with. Sharp cheddar chest has lost most of its original lactose as the cheese was formed, milk has not. Drink a glass of milk=symptoms, eat sharp cheddar, not so much.