My back has been hurting since yesterday.

I woke up yesterday, and suddenly my lower back feels heavy and painful. The pain isn't severe, but when I bend my back, sit down and stand up, or lie down and get up, the pain feels heavy around my pelvis rather than my lower back.

I was fine when taking walks or strolling, and I even walked 10,000 steps, but the pain hasn't gone away, so I'm feeling upset.

What part is deteriorating to cause these symptoms?