I have this constant pain in the bottom of my back where the back and crack meet. everytime i sit, bend, or lie down it hurt immensely. is it serious?

Most likely not: If the pain has been there for more than a few weeks then go see your doc. At your age is most likely muscular unless you recently had an injury or traumatic event. Most low back pain is self limiting and will improve with time. Physical therapy is a great first step. You doc can determine if any other workup needs to be done.