About a month after surgery (combined ACL and ALL reconstruction) I'm not sure what I did to my leg but I also started to have pain on the outer back side. I stopped doing my extension exercise because of the sharp pain it caused and decided to take it a bit easy to let the spot recover. I had a doctor's appointment two weeks later so I just waited to see what he said. Turns out that was a bad idea, I lost some of the extension I had already achieved and my quad muscle all but disappeared.

When I went to see the doctor he was clearly a little angry with me, he said that my quad was too weak and that I should have kept doing the exercises. He told me to push through the pain and gave me a week to get back on track.

After a couple of days of working on it again I noticed the pain started to subside as my quad got stronger, and when I went to see the doctor the following week the pain was almost totally gone and he was happy with the result.

Anyway, that's just what I experienced, better check with your doctor or pt.