Most back pain gradually improves with home treatment and self-care. Although it may take several weeks before it completely disappears, you should notice some improvement within the first 72 hours of self-care. If not, see your doctor.
In rare cases, back pain can signal a serious medical problem. See a doctor immediately if your back pain:

• Is constant or intense, especially when lying down or at night
• Spreads down one or both legs
• Causes weakness, numbness or tingling in one or both legs
• Causes new bowel or bladder problems
• Is associated with abdominal pain or pulsation (throbbing), or fever
• Follows a fall, blow to your back or other injury
• Is accompanied by unexplained weight loss

Also, see your doctor if this is new pain and you're older than age 50 or have a history of cancer, osteoporosis, steroid use, or drug or alcohol abuse.