The muscles in my lower back have been spasming for a couple hours now without pain. what could be the cause?

Back pain: The spasm in the back can be due to lot of reasons, it is mostly seen with back sprain or other related musculoskeletal issues. You can use heating pad, over the counter topical cream such as bengay or asper cream. If symptoms persist then see your doctor.

More than one: The cause can be muscle based, arthritis based, disc based or even fracture based. Stretching both flexion and extension exercises of the back can be beneficial. Your evaluation will look at these options but since you are having the symptoms now beyond the stretching would try depending on your medical status - acetominophen, nsaids, etc as long as they are safe for you.