Could it be something bad if I have back pain?

Pain is a signal. There can be several reasons for back pain including lumbar disc degeneration, muscle inflammation, facet joint arthritis/irritation, sacroiliac joint arthritis/irritation. Rarely a fracture or tumor. A thorough physical exam and diagnostic studies will help determine what treatment will be best depending on your diagnosis. See a board certified pain specialist. For evaluation.
Depends. There are many causes for back pain ranging from pulled muscles to disc herniations/problems to vertebral fracture/problems. Unless you remember exactly what you were doing to bring on the pain originally, it's a good idea to see your doctor for an evaluation.