Muscular,disc proble. If you are having constant pain in the back you should have it checked by your doctor as it may be disc problem.Muscular pain, arthritis or some other condition.
See below. Rest, heat and gentle stretching are a good start. If this has persisted several weeks or is getting worse, see your pcp.
See a Doctor. Back pain, as with all pain, is your body's way of saying "something is wrong". Causes of back pain range from the very benign and innocent such as spasm/strain/sprain to more serious causes. Your doctor can work-up and diagnose the cause of your back pain which will be your first step towards hopefully obtaining some relief.
See a physiatrist . The best medicine for low back pain is ice, heat, stretching, exercise and time. Topical creams and patches may help too. Medicine by mouth should be avoided unless the above do not help. The type of medicine used should be guided by a physician. Sometimes injections may be helpful as well.