How Can A Large Knee Lump Be Treated?

I have had an arthroscopy on my knee for a cyst and tear that was on the right hand side of my right knee. It was now 9 days ago i had the operation and now have a large lump, where the cyst was before. it is big and sticks out about half an inch just around a small area about 1.5 inches in diameter. What is it? It does not hurt all the time but occassionally when walking/ going up stairs and also if it is hit or knocked.