Lactose intolerance getting worse?

I'm 13 and I'm lactose intolerant.. It didn't start until about January. I was never really like this before. Although sometimes when I would drink milk I would get really mild stomach cramps but it didn't really bother me. But now.. It seems like it's just getting worse and worse. Now I can't eat cheese and I know some cheese doesn't have lactose but I still get sick. ANYTHING with even the tiniest bit of milk in it and I get diarrhea. When I have a lot like a few weeks ago I ate pizza (i THOUGHT i was fine to eat cheese then) and later that night my stomach started hurting really bad and i went to the bathroom. It's not like stomach cramps though it's just like a REALLY bad upset stomach. why does it seem to be getting worse?? help!