What Causes Knee Pain With Swelling?

My right knee has been swollen with pain when I move it certain ways. There seems to be fluid on it. It has been about ten days. I started playing basketball daily about 5 weeks ago to lose weight. While trying to bend my leg backwards ( like touching your heel to your butt) I cannot bend the bad knee back nearly as far as the other knee. I can still run and jump and play, so I can deal with the pain. Do you think this is a tear or just a pulled muscle or something?