Suggest Remedy For Knee Pain With Swelling

I am tennis Player and have been recently experiencing pain on my knee. There is a small amount of swelling and discomfort is greatest when stretching and running. I can only bend my knee about 40 degrees without painI am usually able to put weight on it while walking without feeling pain but when ascending and descending steps, there is some discomfort. What could this be and how should I treat it?