Unfortunately, it could be a few things. I'm in your same boat 2 years later. Try getting tested for arthrofibrosis. It could be scar tissue (which is what I think I have). What works best for me is myofascial release performed by a physical therapist. I've had dry needling, cupping and am now using water therapy. It is easier to walk and exercise in water. Some of us are just more prone to forming scar tissue than others. Don't accept the answer that they normally give "wait a year and it will get better." It probably won't. Find a good PT that can perform MFR and see if that helps you. Keep us updated and don't give up hope. Prayers for you that you find an answer.