When walking, i get a very sharp stabbing pain in my lower back. why?

Pinched Nerve/Muscle: Most times back pain that is sudden onset will resolve. He should use anti-inflammatory meds like ibuprofen, aleve around the clock and even tylenol (acetaminophen) to help. If this does not resolve he should see his doctor for a muscle relaxant as well. If it still does not resolve, i would recommend him to see a spine specialist for a full evaluation and testing. He may need to get an MRI to see inside.