It might be worth asking for further tests (from a doctor not someone in a health food shop!). I had to do a revolting test in hospital which involved drinking lactose and being stabbed with lots of needles!
Getting a lactose-free diet right takes a LOT of experimentation. Some people can start to introduce a small amount of lactose back into their diets after a lactose-free period, others definitely can't. I can't tolerate milk or recipes containing milk at all. But other dairy products contain smailler amounts of lactose, such as mature cheese and yogurt. I have a pot of yogurt each day with no problem. You need to start reading labels as lots of things contain lactose, including some boiled sweets! Many pills also contain lactose, but my doctor said someone would have to be very, very sensitive to have a problem with that amount of lactose.
You just need to try different foods out and not get too obsessed with it all. I still sometimes get symptons and can't always remember what I ate that was wrong. And sometimes I just really want that ice-cream or don't want to offend a friend by refusing something they cooked. Basically, the result may be a bit unpleasant, but occasionally going wrong won't kill you!