Sounds to me like you might be suffering from patellofemoral syndrome as a result of muscle weakenss following the surgery. (you may want to google patellofemoral syndrome) Rehab is to engage in quads strengthening in the form of squats (leg extensions can cause further problems due to the stress they put on your kneecap so i'd avoid these) Also strengthen hamstrings and butt muscles. Stretches for your iliotibial band also helps. You may want to look at taping your kneecap so that you can perform squats pain free and therefore get on with the strengthening. You should be able to find a lot of this stuff online. If not I'll see what I can do...see how you get on.