While I ran, a muscle became tight and a little sore

I've noticed during the past few runs, that a particular muscle in my left leg became tight at around 4.7 miles. In previous days, the tightness only lasted a couple of minutes. Today, the tightness came at about 4.8 miles and didn't go away until I was home. As soon as the tightness occurred today, I stopped running and tried to stretch it out. It remained, and I walked slowly the last 1/4 mile to my car. Before getting in the car, I did my usual post-run stretches, but the muscle was still tight. After I had been home for an hour, I noticed the tightness was gone. The muscle is on the back of my leg, right where the ham strings join my buttocks. I don't think it is an injury; probably a muscle that needs more time to adjust to the 5-mile distance.