I've had sharp pain in my back while bending for the past two days. what could cause that? i've had headaches, neck/back pain for the past two years.

Muscle spasm: Most commonly this is a simple muscle spasm that can cause acute pains. If you are prone to other medical issues, fractured bones or even disc herniations may be causing you the problem. Consider seeking medical advice for xrays if the problem persists longer than a week or two.

Acute on Chronic : It sounds like you are having acute on chronic low back pain. Odds are that this increased pain will pass in time. Since it has only been two day. Treat with relative rest, otc meds for pain and no more than 2 days bed rest. If symptoms last more than 2 weeks then go see you doc. If pain is poorly controlled with otc meds then go see your doc. If you have an bowel or gyn issues then go todoc.

Depends.....: At your age it is likely to be muscular in nature. Certain positions can trigger muscle spasms in some people. Try a heating pad and gentle massage. If people have herniated discs the pain can be worse with bending forward, but that would be uncommon in your age. If it does not improve, it would be worth a quick evaluation by primary care and likely physical therapy.