Given the mechanism of injury, and what you are describing, it could be PCL, maybe meniscus, hamstring tendon or popliteus tendon, but these appear to have been ruled out. I think your next course of action should be a referral for MRI investigation. You should be able to get the physio you are seeing to write to your GP outlining what she has done and that she recommends you are passed on for a scan. I think 3 months down the line this is the sensible course of action.