Runner's knee. Was feeling increasingly uncomfortable while running to the point that I stopped exercising altogether, hoping that it would stop altogether. Eventually went to see a physical therapist, who, among other things, diagnozed me with lack of balance and a wrong running style. After a massage and strength/balance exercises I finally think I am ready to restart running.
I should be paying more attention to running mechanics, because once I increase my training amount, improper running can result in an injury.