Bending knee after arthroscopy - 6 weeks on & locking up. Any help?

I'll go ahead & ask here. Can someone move it to the appropriate forum if this forum isn't correct please?

I had arthroscopy on my right knee 6 weeks ago. During that time it swelled up quite bad. My GP said it was abnormal, but as it wasn't hot (& i was seeing the surgeon the following week) the GP wasn't concerned.
Had a bit of a fall out who claimed it wasn't swollen (put it this way, the swelling has now died down and my right knee is 2" bigger than my left, my calf is 1.25" bigger ... but it wasn't swollen(!)).

Then had issues with a potential blood clot after intense pain, but that investigation got knocked on the head. Finally after 4/5 weeks i was able to begin exercising.

I can work through all the exercises given perfectly fine (towel under the knee & lift leg, lay with legs flat on floor, bring heel towards you as far as you can, hold, release).

One exercise is giving me trouble though. If i hold my leg straight, lift it a few inch off the floor and then lower it, when i bring it in (like the exercise mentioned before - bringing the heel in) my knee begins tightening, as though it wants to crack. Everyone will have had this feeling at one time or another - their knee, ankle, elbow etc). So i bring it in slowly & the pain gets unbearable, so i have to let my leg extend out once or twice & bring it back. The "point of cracking/locking" then disappears.

Why is it doing this?

* Apparently my kneecap was displaced. I've had the back of the kneecap shaved. I've also had the plica (is that correct?) membrane removed.

When i asked what purpose the plica served, i was told "none". I find this difficult to believe - surely everything has SOME purpose?! So i tried to google it, but all i find is "plica syndrome" & not what the plicas actual purpose is & the results of not having it.