What to do if my lower back hurts very bad. what should i do?

Depends on several : factors as to injury related, associated fever, chills, sweats or unexplained weight loss or loss of appetite, other symptoms like neurologic -weakness, numbness., bowel/bladder control issues, is the pain worsening as examples. If any of the above see your doctor now. If not & pain of short duration try limiting activity & OTC meds if no reason you can't use them. If no better, then see doctor