I have a pain in my right leg on the inside of my knee which is sore but I don't know what it is?

I have had it for a couple of months now and picked it up after running and training. It is at the side of the knee cap on the inside of the right leg and don't have a clue what it is and I don't want to run because it might get worse. However it doesn't hurt when I run its always after, I can not go to the doctors because i'm in the process of joining the Royal Marines and don't want anything on my medical records which might hold me back. Someone please help!! How do I get rid of it?