Hay, oo naman normal yan during pregnancy, ang uterus mo lumalaki at nag-stretch, kaya parang matigas talaga lalo na sa gabi pag naka-lie down ka ng matagal, tapos umuumbok pa dahil sa baby and amniotic fluid! One of my mum friends said ganyan din siya around that time, nag-relax lang siya with warm compress and proper positioning, pero since you're noticing it's quite pronounced, better check with your OB just to be sure walang dehydration or anything else going on, hindi naman rush pero good to mention sa next checkup mo, yan lang ang safe route especially at 18 weeks pa lang.