pain in back of knee

I was playing softball the other night. After running to first base and extending my leg to touch the bagI got a little pain in the back of my knee. After a period of time it began to get more sore. The next morning it was really sore. As long as my knee is bent I don’t really feel any pain, but when I straighten it out the pain starts. It gets worse when I step on it. Can you possible tell me what this is and treatment for it.