There is a thing called secondary lactose intolerance, where as a result of some kind of "insult" (usually a bout of gastro or a large amount of a protein that you are sensitive to) the bowel lining is damaged and becomes much more sensitive to gas production. It should get better when the "insult" is fixed though.
Is the lactose free milk UHT? It may be that the UHT treatment (which is a more aggressive heat treatment than normal pasteurizing) alters the milk proteins in such a way as to make them less allergy-inducing for C. Try a tiny bit of UHT normal milk and see what happens!

You could also just give her lactose and see what happens - if you can get hold of the dummy pills from a pack of OCPs and crush a couple and see if they provoke a reaction.....They contain a few milligrams of lactose each - about the amount in a ml of milk.