What Causes Sore Knees?

Hi, I have been suffering from sore knee's for over a week now. I am 24 and play football (female). I find it hard to explain the problem with them. When i bend them to go to sleep I cant but them straight again with out feeling some kind of spasm. It a weird kind of spasm pain that is like a pop and crack in my knee's along with the pain. I take vit.c cod live tablets on a daily occurance. I feel like I am restricted to what i can do some as bending down and moving. What could be the problem?
Thank You.