Feeling Adrenaline Rush In Right Leg Which Recently Got Operated

Feeling Adrenaline rush in right leg which recently got operated for a Torn ACL. I am one month past operation and still not bearing weight on the leg. Whenever there is an adrenaline rush in my leg, there is some pressure on my knee cap as it gets pushed down, there is sudden pain but i feel relaxed after this. Whats is the Cause of this rush?