Leg injury from running?

What have I pulled/ injured in my leg? I was running through a forest dodging logs and rough terrain and such but I don't think I twisted it and I didn't fall. It was an easy run to. We were about to finish and my leg started to feel a bit tight. When I started walking, it felt very awkward (it's my left leg btw). I can't quite pinpoint the injury, not sure if it's a hamstring or something else, but the pain seems to be somewhere on the back of my leg in the thigh area, kind of near the ocks too. it feels really akward when I walk/run, and there's some pain but it seems to happen randomly and it's only minor.