My first thoughts:

I believe at the rate of your running, and the insidious type of onset there is some form of muscular imbalance going on.

Studies in hockey players for example, discovered that 10% of all sporting injuries in this group occur at the groin. The reason being the imbalance that occurs with the type of force push-off, and imbalances that occur.

If one took an EMG of all the muscles around your hip and legs during running, it would become very apparent that some muscles are firing more rapidly than others. (this is not a common test btw)

What does the body do? Some parts tighten/spasm, or improper or unusual glide of muscle groups and muscle fascia occur.

Of course, this may not be the case at all, but I would get it checked out,