Possibly acute articular cartilage damage under the patella otherwise overuse retropatellar inflammation. If it doesn't settle down like retropatellar pain should then I will need an MRI. Had MRI and it showed cartilage damage so the next step is an arthroscope to have a better look and hopefully resolve the problem.
Had arthroscope and knee is improved but quite possibly will never be 100%. Have been able to return to training but still need to do further rehabilitation and strengthening to improve.
Strength and stability work plus time.