I have the same problem in my back, in the same area you desribe. Is it stiff and sore in the mornings? The first thing you gotta do is see your GP and he will make you get an X-ray and refer you to a PT. If your X-ray comes back clean, which I thihnk it will cause your not in bad pain, then its a muscular or tendon problem and not a spinal or sacroiliac joint wearing problem.
When you see the PT, they will probably start you on pelvic execises, to "re-activate" your pelvic muscles. If this is a cronic pain, which you said its been annoying you for longer then 3 months, then I would ask the PT to do acupuncture. Acupuncture is great for waking up scarred muscles.
Currently I'm getting acupuncture done. Since we have problems with the same muscle, lets keep in touch with eachother and updated on our progress