Sorry to hear of this trouble.
I would recommend that you see your doctor right away.... you may even wish to go to Urgent Care or to the ER if you are having fever
Without an exam I can not be sure but I would be concerned with infection that will need antibiotics and possible drainage
of the knee joint. without treatment an infection, if present, could progress to the joint and bone. this is a much more serious situation.
Again, this may just be inflammation however it would be in your best interest to be evaluated as soon as possible