HELP! My left knee is so swollen. It is very swollen, warm, and feels like it is full of fluid in the BACK of my knee. I feel PAIN in the hamstring, next to the tendon that runs behind the knee (the big tendon that sticks out...), it feels so tight, swollen, and is a sharp pain the first few steps of running...like a knot, and limited movement. HELP! What is going on????? Tendonitis???? Knot????? Just tight????? What should I do???? Thanks.