Thanks for posting your query.
According to description , it seems you might develop synovitis
knee after your previous injury.
Synovium secreates more fluid after it get inflamed. So that you developed pocket of fluid over your knee.
It needs aspiration, compression by creape bandage and management of knee injury
I suggest you to consult your doctor for a detailed examination and treatment plan according to that.
Wishing you good health
Dr Saurabh Gupta