Real Runners said:Hi Nellie
I would encourage you to see someone about it - maybe a sports physio, it sounds like it Runners Knee, very common and its not your fault! Are you icing the area? Taking any anti inflammation tablets? Massage? Definitely get some advice asap.
It sounds like Runners Knee (9 times out of 10 - pain behind the knee is runners knee) however it could be something on the lines of Baker's Cyst, which is a real pain and requires an op to remove. For the sake of £30, go see a specialist it will put your mind at rest! Good luck Nellie.