My lower back hurts when i'm sitting down for more than 10 minutes or when i bend over. what could be the reason it hurts and how can i prevent it?

Disc dis. or arthrit: Commonest cause of back pain is degenerative disease of spine which is arthritis. Bulging discs may also cause similar symptoms. See a neurologist or orthopedic.

A disc: There are a couple of reasons why this could be happening. The symptoms you describe are typical for discogenic pain (pain originating from a disc). There is increased pressure on the disc with prolonged sitting or with flexing forward. It could, also, suggest be muscular pain, mechanical in origin. Would suggest you see a physiatrist.

Large list : Of causes. Back pain is pretty common and often no cause is found. Here is a good article to get you started http://www.Mayoclinic.Org/news2010-mchi/6044.Html.