Your weight and present treatment certainly could be contributing to these symptoms even if they are new. I suspect I have a herniated disc in my lower back that is causing pain in my left buttock. See your family doctor or an orthopaedist for a diagnosis. Herniated discs can be treated conservatively with limited activity, medication and physical therapy. Slowly, the condition will improve.
Surgery is sometimes necessary.
You didn't mention how long you have had the pain, but if you're writing to us you must be concerned. You should find out for sure what's going on with your back. Check the calendar.