Eh, this is quite normal lah during pregnancy, your body is changing and the weight distribution puts pressure on your back, especially when you've been lying down for hours. A few of us mum friends experienced the same thing, habis tidur je sakit, then it improves once you move around sikit. Since you're 15 weeks, I'd suggest checking with your doctor about it to make sure everything is okay, tapi in the meantime try adjusting your pillow or sleeping position, some of us found sleeping with a pillow between the knees helps. Do you sleep on your side? That usually helps during pregnancy.