Your center of gravity will gradually move forward as your uterus and baby grow, which causes your posture to change and causes back pain
It can be due to hormonal changes / poor posture / additional weight
Next Steps
Apply heat with hot water bag
Health Tips
Try to maintain good posture
Sitting or sleeping too long can make your back hurt even more
Try not to stand too long either
Sleep on your side with pillows between your knees for support
Avoid high heels
Wear comfortable footwear and clothes
Do physiotherapy
Regular walking
Yoga will also help
If these all things not working then consult Gynec/ Ortho for further management