The reason could be one of several things, ranging from minor to serious, that only diagnostic imaging and other medical exams can determine. It could be muscle pain, bone pain, nerve pain, etc... I suggest you see your doctor about this. Take OTC pain meds, use a a heating pad, and use products like Ben-Gay, IcyHot, SalonPas, etc... If none of that works, you'll have to ask your doctor for prescription medications. Good luck!