#1-do you have full knee extension?  I’m assuming that’s a no based on what i read.

#2-I’m also assuming by “PT” you mean physical therapist, and if so, did he/she do a full eval to check ROM of the hip/thigh/lower leg/ankle/foot? That would help key in with where to start.

#3-Does said “pop” hurt?

Pain being the symptom, unless you abruptly did something traumatic like a knee sprain during running or jumping (at any point), I would say its probably assocaited with some sort of dysfunction up or down the chain. Address ROM of the muscles surrounding hip/knee/ankle/foot. Don’t torque on the knee joint, like i said, chances are its due to lack of ROM of the surrounding musculature.

The two big ones i would see about:
Your hamstring flexibility on each leg separately (where you’re unable to keep a flat back).
You hip flexibility, predominantly the rectus femoris.

I used to have a “click” near full knee extension (not painful), which could also be called a “pop”, following my most recent ACL surgery. I found addressing hip weakness and mobility in the musculature surrounding the knee and hip to be most effective in eliminating this and restoring proper ROM to my knee joint.

Interesting. Sorry, i keyed in on the knee rather than the sitting pain. That honestly sounds more like a sciatic nerve irritation. Just a guess, having seen a couple people have similar issues/pain (sans knee “pop”). The “pop” may have been unrelated or just due to general tightness or the quad/ham and the patella glided back into proper position? Speculating of course, but i’ve had the same sensation just without the leg pain.

I’d still advise working on hip abduction strength, and as you already were doing…loosening hams and quad (and calf/ankle/foot).

Good luck. i hope it doesn’t occur again!

Now that is interesting.

I assume by this point you ruled out any other activities from the day of the onset of posterior chain soreness, or the days immediately prior, as a culprit?