Walking is fine. Even running on the flat ground is ok but deep knee bends hurt. I can’t do quick stops or hard cycling or stairs.
It’s a generalized ache/pain, kind of on the top on the knee, inside the knee and all around? The whole thing feels tight and poppy. Could this be cartilage tear?
Could be 10 different things from your description, but only a qualified PT or orthopod will be able to tell/know for sure!
OK, first things first, I’m not a doc or a PT so the usual caveats apply.
Based on the info you gave (i.e. pain with loaded flexion), as Gary said, could be a number of things including:
Meniscal tear
Chrondomalacia
Patella Tendonitis
Bursitis
ACL sprain
My first thought would be Chrondomalacia, as that usually presents
a) as a chronic condition
b) pain while ascending and descending stairs or squatting
c) recurrent swelling around the knee-cap
d) a grating sensation on flexing and extending the knee.
The only thing that I would tend to eliminate would be a collateral ligament strain because usually these present with pain on full extension (not flexion).
All that said, I am just a regular guy (not a doc or PT) and IMHO, you don’t want to mess around with diagnoses on ST. The knee is way too valuable for a triathlete to guess on this stuff.
Out of curiosity, what was the onset of the problem?
Alan
Not enough PLYOs. ;^)
(Sorry, not an ortho, but may just be anterior knee pain syndrome.) The usual treatment, if that is the case, is quadriceps strengthening and a little rest.
Incidentally, chrondomalacia is more common in women (not sure from NYSLIM??) and athletes with malalignment/patella tracking issues, typically valgus (knock-kneed).
I had somewhat similar symptoms to what you described with a meniscal tear, only difference was, my pain was much more point-specific. I could clearly identify it as coming from the medial side of the knee. Incidentally, the cause/onset for me was a long ride with my cleats in a funky position (too much external rotation).
Bottom line, please see a good sports doc.
Hope this info helps.
Alan
Yes, and of course, begin plyometric training immediately - single leg weighted depth jumps from about a meter or so are just what the doctor ordered 
JUST KIDDING!!!
Alan
I’ll guess IT Band Friction Syndrome, but I’m neither a doctor or have any education on the subject.
However, I am psychic.
Also, make sure that you are stretching your calfs and hamstring- this can relieve some tension in and around the knee area. Longer term, strength the quads and see a sports doc; maybe some ART?
Thanks for all of your replies. I will go get checked out if it doesn’t clear up soon.
PS - funny comment regarding the plyometrics
FYI - this happened in a silly way. I was running into the movie theater from my car and since I am faster than my friend, I started running like a goof-ball so I would go slower. I was sort of bounding from one leg to the other (I guess you could call my action plyometric) and then my left kneed just sort of locked up and it started to instantly hurt in a catastrophic way.
i’d see a good sports doc…trying to diagnose something like that over the internet will be nearly impossible. You need someone knowledgable to actually manipulate your knee and check the stability of the joint.
If it’s limiting your activity, it sounds like the pain is fairly severe and worth the inconvenience of a dr’s visit.
Best of luck.