I had arthroscopic knee surgery in 1995 and reconstructive ACL on the same knee in 1997. You definitely need your basic rest, ice, compression and elevation (a.k.a. RICE). Everyone heals and adjusts at different rates. There is a chance you began walking on it too soon. Your leg will hurt or feel tired especially after physical therapy, but that is normal. Try asking your physical therapist and your doctor about this problem. Have the same doctor who performed the surgury take an xray of your knee if the pain becomes unbearable (where it has an impact on your regular daily routines). Good Luck!