Eh jangan worry lah, 15 weeks tu normal jer perut kadang keras kadang lembut, itu ligament stretching dan baby pun kecil lagi so the belly relaxes in between! A few of my mum friends said the same thing, siap takut pulak thinking something's wrong padahal semua okay je. Bagus you're going for your scan esok, doktor akan confirm everything is good, tapi honestly this back-and-forth firmness is super common at this stage so lepak jer.