Aiyoh, sakit belakang masa pregnant or postpartum ke? Either way, better check with your doctor first especially if it came on suddenly and you can't move comfortably, could be something that needs proper attention. A few of my mum friends had similar issues and turned out they needed physio or adjustments, so don't tahan-tahan ja. In the meantime, try sleeping on your side with a pillow between your knees, and avoid heavy lifting lah!