I am 4 weeks post op on an acl reconstruction. i have a sharp excruciating pain in my calve muscle of the same leg for about 2 weeks.why?

Hard to say: In my opinion, the most immediate concern would be to make sure you don't have a blood clot - highly unlikely after acl surgery, but an ultrasound can be done to evaluate for that. Other than that, it is certainly possible that a muscle or nerve was irritated by the surgery, and should resolve over time. Do see your surgeon for further evaluation.