Help With Leg Injury, Running

Hey guys i wasnt sure if this is the right place to put this but here goes…

Ive been runnig for quite a while now and have a weird tightness in the back of my right leg parralel to the knee after running, i can feel it a little when im running. There is no pain but stiffness/tightness after, it doesnt bother me when im not running but if i straighten it fully it feels slightly tight. It feels like its in the middle of the rear of the leg. I have increased the running lately so could it be overtraining?

Just any help from people with experience in this will help so much. It may be that im favouring one side as i had a weird strain on my left heip a few inches down so i may be leaning to one side even though the hip doesnt hurt, could be subconcious :S, hmmm…

Im hoping i dont have to stop running as Im training for the royal marines so i need to keep up the running. I know a guy who had something similar but was a weak tendon in the back of the leg but i havent seen him for a while to clarify the symptoms, any ideas? any help will be appreciated!!!