I hope someone has some suggestions.

I have been riding for 5 years, but this spring I started having pain in my right leg. It starts on the out side of the leg, just behind the knee. Feels sorta like a cramp, but seems just to tighten up. It is more pronunced the harder I push and usually comes into play after 25-30 miles. It moves up my leg twards the outside of my hip and into my back. On hard rides it gets unbearable and I have to slow or stop.
Could this be an overuse injury from training too hard in the spring that I haven't rested? Or do I need to strech a specefic muscle group?
Ideas??