What can cause lower left back/side pain?

Some thoughts... For a young guy who's otherwise healthy, we think of lower back strain (muscle strain) as one of the most frequent causes. Other things that can give pain in that location - kidney stones, gas, constipation, and less frequently kidney infections, diverticulitis. If you're having a fever, numbness or weakness in any of your legs, or problems controlling bowel or bladder, see your doc asap.