I have undergone an acl replacemet surgery 3 months ago.i have been doing physiotheraphy for 2 & 1/2 months.i still get a pain in the back of the knee?

Normal: If you are having posterior knee pain 3 months post/op acl reconstruction this could be a normal 'discomfort ' , esp. If you haven't regained complete full extension in your knee.This pain should be improving , if it does not or worsens see your treating ors for evaluation.

Not uncommon postop: Postoperative swelling within (effusion) and around the knee is not uncommon out to 3+ months after major acl reconstruction. It also depends upon the status of the remainder of your knee. Assuming the radiographs look fine, and no problems were noted at surgery, there should be nothing other then fluid pushing out via the back of the knee. The posterior aspect is more sensitive and pain is noted.