Tight nerve and pain behind knee

Hi, I'm looking for some friendly advise please. I recently returned to my first love, cycling, but after my first spin of 40mins behind my left knee got very sore. I rested it for 7-8 days and then did my second spin and it got very sore after 10mins. The pain subsides after 2-3 days. The pain is right behind the knee, around the popliteal muscle.
I have noticed when I do a slump test I can't straighten my leg. But when I do a second and third repetition of the slump-test I can straighten the leg. Thou behind the knee will get a bit tender. Also, regardless of slumping the upper body and head/neck I get the same results - not being able to straighten the knee on the first rep, but I can on subsequent reps. But the knee will be tender.

When I release the dorsiflex I can straighten the knee, but no effect of releasing tension from upper body/neck. Could anybody please shed some light a to what is going on.