Your symptoms seem to be related to either of the two causes :
firstly it may be due to bursitis
which means inflammation of the bursa around the knee.
secondly it may be due to localised collection of blood also known as hematoma
In either case you will have to go to your operating surgeon and get yourself evaluated.
kindly get the swelling aspirated and send the fluid for culture and sensitivity report.
If only blood is aspirated then you need not to worry. Rest , ice packs, compression bandaging will do the trick.
I hope my advice has helped.