You may have a herniated or slipped disc. Between every bone in your spine there is a rubbery disc that acts as a cushion between the bones; if they slip or are swollen the nerves in your spine go haywire, thus the pain. You need to trace back in your memory the past few days to find when, where and how this happened, it will help tremendously.
The bad news is that this pain will not go away easily. The good news is that you are Fifteen and your body can bounce back faster.
I think you should have a discussion with your parents to see both a doctor AND a chiropractor. Usually doctors will want to cut you up and give you pain medication, thus masking the problem. A chiropractor, if it can be done, will with time, move the spine back to where it needs to be. I myself have a very bad back, and starting to get arthritis in it, but with the help of a chiropractor it keeps the pain in check.