I'd say you're leaning more towards having IBS rather than lactose intolerance (I have both, including crohns disease)...have you had any GI tests done? Like a colonoscopy, blood work, stool samples, ect...first and foremost I would demand your doc (GP) to send you for these things because it's not normal to have D, there is obviously something going on with your GI tract and it needs to be properly DX so you can get appropriate treatment.
IBD (crohns or ulcerative colitis) could very well be the culprit, there is inflammation involved with either of these diseases. With IBS there is no inflammation involoved which is why you need tests done to hopefully get a proper DX.
My bum is broken....there's a big crack down the middle of it! LOL :)