What Causes Severe Pain In Back When Diagnosed With Diverticulitis?

My doctor believes I have diverticulosis. (I had a CT scan earlier in the year when on holiday in Australia. Nothing showed up but was advised it wouldn't show up problems with the colon) I find that when I walk for long periods I do get a pain in the back. Also sometimes the pain in my left side seems worse for no apparent reason. How much discomfort should I put up with before asking to be referred to a specialist?