Wahhh first baby excitement! Perut mengeras tu biasa je, especially kalau baby bergerak atau uterus mu tengah stretch, some of my mum friends cakap at 18 weeks they dah start rasa Braxton Hicks (fake contractions) but that's normally later, so yours is probably just normal muscle tightening or baby moving around. But honestly, since this is your first time and you're not sure, better check with your doc or midwife next visit, they'll listen to you and confirm everything's good, and then you boleh relax!