Aww I know that feeling, so uncomfortable kan, at 29 weeks kau memang getting heavier and baby's taking up more space, so back pain and that all-over soreness is super common lah especially if first pregnancy u tak experience. A few of my mum friends say their second pregnancies were worse because their bodies dah "know" the weight, plus ligaments are already stretched. Try changing positions often (side-lying with pillow between knees helps), warm compress on the back, and maybe a maternity support belt, really can make a difference. But since it's pretty persistent and affecting how u sit and lie, best la check with your doctor or midwife just to make sure everything's okay, better safe kan?