Eh, perut memulas tapi bukan nak berak memang boleh jadi a few different things lah, bisa constipation (even if you don't feel like pooping!), gas, atau ligament stretching sebab baby growing. At 13 weeks you're right in that phase where perut start expand, so itu normal je sometimes. Tapi kalau sakit tu dah severe or terus-terus, or ada bleeding, best check with your gynae cepat-cepat yer, better safe than sorry during pregnancy! One of my mum friends kena gejala serupa and doctor just reassured her it was stretching pain, so don't worry too much lagi.