The back of my left leg, knee area is sore. i have a desk job, i don't do strenuous physical activity to warrant pulled muscle. what's the deal?

Could be a number of: Possibilities; 1) muscle strain which can occur even in a sedentary person, after low level activities like climbing stairs, or a rapid walk, or running after a child, etc; 2) could have bumped the back of the leg/knee into something; 3) could be leg clot, which can occur in a sedentary person. Is there swelling, tenderness or breathing problems/chest pain...... See your pcp the last possibility.