Ay, back pain time pregnant memang common lah, especially bila dah masuk trimester ketiga! A few of my mum friends complained about the same thing, one of them said it's because the weight of baby puts pressure on the lower back. Definitely tanya doctor or midwife pasal ini, especially if it's getting worse or jadi sharp pain, sebab they can give you proper exercises or suggest physiotherapy yang aman for pregnancy. In the meantime, try rest properly, avoid lifting heavy things, dan maintain good posture when sitting, senang je tapi memang buat difference!