Sounds similar to an injury I've been battling for years. If so, it would be called something like a chronic hamstring tear. This is not to be confused with the hamstring tears that sprinters/jumpers get. It is an overuse injury that 99.9999% of US doctors will misdiagnose. Gerard Hartmann has written some articles on this injury and I've been lucky enough to have him treat it. Basically, its an accumulation of micro-tears at the top of the hammy where it attaches to the pelvis(?). Once a small tear starts, possibly from a speed workout etc., the problem only increaes, the small tears heal and then retear, continuosly getting larger. The radiating pain is fron the surronding muscles taking on extra workload, from the hammy shutting down, big problem is that the longer this continues the weaker the hammy becomes, basically atrophy sets in, compounding the problem. While the hammy was the root of the problem, the aches and pains were all around the ass/hip area, it causes sciatic pain and can create IT problems, but those are only symptoms of the real injury. This is the injury that basically ended my competitive running.
Ways to fixed it: Must take a certain amount of time off from running and possibly even cycling etc. Deep tissue massage workout and multiple physical therapy tools( ultra sound etc.) need to be used in order to promote blood flow in the area, as it is essential to begin the healing process. This is a several week scenario. If a person with this injury thinks they will be back running in a few weeks they will only end up repeating this process at a later date.
Once the pain seems to be gone, the real work begins, a complete core strengthing program needs to be implemented. focusing on the hips, hammys, lower back, and most importantly on the lower abdomen. If you have a large curve in your lower back from your lower back muscles being stronger than your ab's, then the tension on your hammy's will never be released and you'll be susceptible to reinjury. This is a little tough to describe in writing.
If this sounds like your problem let me know and I'll give some more advice.