What can cause sudden lower 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.

Strain: The most common cause of sudden low back pain is a strain. This is usually of the muscles and the ligaments. Certainly other tissues can cause sudden low back pain such as a disc tear or disc herniation. The good news is that most acute low back pain goes away within a few days with rest, ice and over the counter anti-inflammatory medications.