I have pain in my back after lifting a box?

Lumbar muscle strain. Back pain is the one of the most common diagnoses seen in primary care. The initial treatment for lumbar (low back) strain is conservative: tylenol, (acetaminophen) nsaids (aleve), ice (30 min. On/30 min. Off), heat (30 min. @ a time). Bed rest should be limited. 'hands-on' therapy is helpful: physical therapy, chiropractic, etc. Xrays/mri's are not needed initally unless severe leg weakness, bladder/bowel change.
Muscle sprain. Back pain after lifting could be due to muscle sprain. If pain persists for extended length, see your p.C.P to evaluate for any disc disease. Early activity and therapy is advised.