I had stopped doing power cleans because when I did, the rear/lower part of my left knee would have a mild ache (tendonitis?).

I just discovered that I can create the same mild ache by simply sitting on the floor attempting to fully straighten my left leg against the floor. Both legs can go completely flat against the floor, however the left leg aches when I do this, (and for a while after trying as well). So I am guessing this is the source of my problems with doing power cleans, and sometimes after squatting as well.

I could be wrong, It doesn't seem to me like an injury, just flexibility? Whats the solution/protocol for this?

(Although that said, I do also have ongoing minor planar fasciitis in my left foot, perhaps the issue is connected)