Why is my left knee in pain?

Yesterday, I went to the gym and was playing basketball. I think I landed kind of ackward, and I felt a twinge in a muscle behind my knee. The next day, I had pain throughout the back and a little in the front of my knee. I can walk and I feel pain when I put a lot of weight on it, and when I bend my knee I feel pain. I feel a little pressure in the knee as well. I hope this isn't the sign of a major injury. What do you think?