What happened to my knee?

About a week ago I was playing tennis and doing my morning workout. But after that day, I notice pain on my right knee. I have never had this pain on my knee before. Whenever I bend my leg, my knee feels like I'm stretching it and feel like it can snap. I'm sorry it's difficult to explain. Do I have this pain because I didn't warm up on that day before my workout? or maybe I just took a wrong step while playing tennis? I don't think I broke my knee but maybe a sprain? Please help..thanks!!