Oo, normal yan sa pregnancy especially around that time! A few of us mums experienced lower back pain starting around 18-20 weeks kasi ang weight distribution talaga ay nag-shift, plus ang hormones are loosening everything up. Mine got worse towards the third trimester, pero it helps to wear a maternity support belt, do gentle stretches, and maintain good posture kahit medyo mahirap na ngayon. But since you're asking and it's bothersome, better check with your OB para sure walang ibang cause, lalo na since pregnancy naman talaga changes a lot sa katawan natin!