What can cause your back pain?

Many things. Lower back pain is very multi-factorial. It can be caused by muscle spasms, muscle/ligament tears, disc disease (bulging or herniation), and facet (joint) inflammation. The cause of these problems can include weight gain, trauma (induced by accident, lifting, too much exercise, etc), or poor core strength. As your doctor more about your specific case.
Back pain. Most common cause is muscle strain (pulled muscles). There are several other spine related causes such as dis herniation and arthritis. Non spine causes include kidney infection/stones, gallbladder stones, pancreas inflammation, ulcers , etc.