Can a pinched nerve behind the knee cause the medial head of the gastrocnemius to stick out? the medial head sticks out in a straight line, very hard.

Be systematic: Nerves generally don't get "pinched" in the popliteal fossa. In any case, a "pinched" nerve would likely cause weakness of a muscle, not hardness or spasm. Baker's cysts, which occur in the popliteal fossa, usually aren't hard like bone. Something that feels hard is likely to be bone. So maybe something bony deep to the muscle is pushing it backward/outward. See a doctor and have it examined.