Eh sabar² la dear, back pain macam ni memang common masa trimester ketiga sebab baby dah besar dan berat positioning berubah, plus ligaments stretch! A few of my mum friends complained the same thing around week 33-34, tu jugak time when baby's head dah settle lower. Try guna maternity pillow atau lean forward sikit bila duduk, boleh juga try warm compress atau gentle stretching, tapi kalu tak improve atau parah sangat, better check dengan doctor cepat² tau in case ada something else.