Knee/hamstring discomfort but not when running

Ever since I ran a trail half about a week ago I’ve had a slight discomfort / tightness down my left leg. I usually notice it in my hamstring or the side of my knee but only after I’ve been sitting a while or during the first 60 seconds or so of running. After that it doesn’t bother me really at all while running.

I was thinking maybe I just used muscles in the trail race that I don’t normally use (since I don’t run elevation like that typically) but any insight / tips would be appreciated!!