Weak knee / pain in one posiiton

Hi guys / gals,

Really hoping someone could shed some light on what seems to be an issue from almost nowhere.
My background - triathlete focused training, 70kg 173cm, I have previously had a knee op - arthroscopy / debridement of patellar (Right knee).

I crouched to chat to my little girl on Sunday and upon getting up (after 5 minutes at a guess), my left knee ached, and just assumed it would wear off, yet 5 days later I am still experiencing the same pain / issue. It only hurts / is not stable when ascending stairs / after crouching and getting up - and more significantly at about a 90 degree angle +/- 15 degrees. Any force through it at this angle gives pain and what appears to be weakness/unstable feel. After that there is no issue - i.e. I could hop using calf muscles etc but certainly can't squat. The other attribute is the patellar, it seems to be off-track and cracks back into place with nearly every extension.
There is no pain in any other direction no mater which way it is manipulated, just this straight leg extension action. There doesn't appear to be swelling and is not tender to the touch either.

It feels like a simple strain that has not rectified itself. It feels nothing like the previous knee issue I have had.

The pain is hard to pinpoint exactly where it is, but feels like the back of the leg, and to both sides of the joint, but not directly outside.

By pulling the leg back, completely adducting it, it does feel tight and as if there is some swelling but it doesn't seem to be noticeable and feels tight over the patellar.

Any ideas of the source of the problem and course of action to take?

Really appreciate any help offered.

I am going to give it a while to see how I get on, but I do need to be active during my job, but walking about is not an issue whereas manipulating stairs is, especially after the first 3 steps!

Failing self-healing, I am probably going to see if my local physio or Doc can aid me.

Thanks guys, I need to get fixed to do the Triathlons I've booked in for!!

Tony