In reply to partz:
The back of my left kneecap, inbetween the two main (tendons?). The pain mostly begins if I point my foot downwards and ceases if I stop the movement. Theres no swelling or obvious signs of deformation.
Based on the above it could be the origin of the Gastrocnemius muscle, which is plantaflex the foot and knee flexion which are both actions involved with rock-overs.
Try some first aid (R..I.C.E) whilst you make an appointment with a physio or sports massage therapist.
Rest, Ice (4 minutes on 90 minutes off), Compress (with your own fingers and follow the muscle down) and Elevate (often as you can).
Take lots of fish oil to help with the inflammation.
Rest it. Listen to your therapist.
Good luck and get well soon.
PS, rest it, the rock, boulder will still be there.