Hi toner,
Further to Catherine's reply, it does seem that more people than ever before are lactose intolerant, hence lactose-free milk and other dairy products being so readily available in most supermarkets these days. Why not buy a carton or two and try it for a week. If your symptoms reduce (or disappear) then you will know the answer. But also seek your GP's advice, especially if your symptoms don't reduce on lactose-free milk.