Constant knee pain...what could cause it?

I am 20 years old and I have been having pain on the medial (inside) part of my knee for the past year...it went away for a while and now its back, worse than ever. I did not have any specifiic injury however I wear high heels for my job and I am on my feet almost all day and I used to do dance. I am guessing it is from one or both of those two. I have been to 2 different doctors and had x-rays as well as 2 MRIs done. They still dont really know what is wrong. They keep saying it "might be" pes anserinus bursitis/tendinitis or plica syndrome. They said there was some fluid in the bursa based on the MRI, but there is always a small amount of fluid in a bursa isnt there? Anyway, I have had a cortizone shot in the bursa and it did nothing...typically that cures bursitis, or atleast relieves it. I dont really think it is bursitis since the pain is constant and it is like a very sore ache...also, the pain is not really in the right spot to be an inflammed bursa. Where I am experiencing pain is right below the medial(inside) joint line of the knee...the knee is slightly swollen in that area, also. I have tried physical therapy and stretching and it has been to no evail. They also said it could be plica syndrome, however, I do not have any problems going up and down stairs and my knee does not click...2 clic signs of plica. I have no idea what is wrong with my knee and the doctors dont seem to either. please offer any help you can. I am going insane here.