Just had treatment for a similar issue, mine occurred when running/rugby training, I've had a pretty comprehensive injury catalogue, and two serious shoulder reconstructions, which was the precipitate for my hip/back pain according to my physio. The explanation my physio gave me is this the tight muscles in my lower back (which are a result of muscular imbalance further up my back, but could be a stand alone problem in your case) are pinching the sciatic nerve which in turn causes spasms on the muscles over my sacroillaic joint (look it up on google images) which again pinches the nerve lower down. This gets worse with heavy or repetitive movement, which stands to reason. My problems are probably a lot more entrenched, a lot of years of semi-pro rugby and serious shoulder issues have made me an injury prone guy! But after a couple of months of treatment I'm starting to get better. One thing I found helped was sports massage and dry needling (a bit like acupuncture), so maybe they're worth a try. A simple exercise the physio gave me to do towards the beginning of my treatment was to sit on the edge of my bed, with the unaffected leg foot on the ground, and extend the affected leg out so it was straight, with my hands cupping my ham strings then lean (or hunch, as she described it, saying it was the only time she'd ask me to use poor posture) into the stretch while moving my foot back and forth as if doing a calf raise. I would do this for 3 sets of 1 min. It really showed me it was the nerve not muscular, as when I moved my neck into the stretch I felt the nerve twitch a little (or the resulting pain from it being moved at least). Hope this helps and isn't TMI!