Im 38 weeks pregnant today.
Rest on your hands and knees. Typically back pain is caused from the baby being face up and pushing on your spine. If you rest with your belly hanging/unsupported while on your hands and knees it forces the baby to turn over and move off of your spine.
Lower Back pain is normal during pregnancy I've had it last few weeks too it comes and goes it's just due to extra weight cause if baby or just your posture changing some people tend to slouch when pregnant! me being one of them I wouldn't worry unless the pain is excruciating... How far along are you?
What could it be...labor?