I Have So Much Pain Behind The Knee, It Feel

i have so much pain behind the knee, it feel like tearing, and all i can find when i touch it and try to rub the pain away [which i can't ] i find a little pee size bump on the tendent on the back of the knee. ibuphron helps for awhile, the pain is soo bad it keeps me up at night. what do you think it is thankyou