Sakit pinggang dalam first trimester memang biasa sangat, but since you've had a miscarriage before, I totally understand why you're worried, please do mention this pain to your doctor at the next scan so dia boleh check properly and set your mind at ease. Rahim yang menebal is a good sign, and your doc will monitor closely, so try to rest bila boleh and don't carry heavy things lah. Semoga this pregnancy goes smoothly for you this time around, stay positive okay!