I've felt bloated for last 3 week but had constipation got flatulence got lower back pain and cramp in stomach have i got ibs or lactose intolerance?

Could be either.: Try keeping a log of everything you're eating, your bowel movements and what else is going on in your life. Lactose intolerance typically causes gas and sometimes diarrhea and crampy abdominal pain, but not necessarily back pain. IBS (irritable bowel syndrome) certainly can cause these symptoms, but is a diagnosis of exclusion. You should consult with your doctor to help you figure this out.

Gluten sensitivity: is a third possibility. I suggest you see your doctor and ask about all of these. You need a good exam to start with. You may need to have some blood tests done if this is a recurrent problem.

Both are: possibilities as well as gluten sensitivity and other causes so see a GI.