Aww sayang, tu likely Braxton Hicks contractions, perut keras tapi tak sakit, memang normal especially kalau dah 18 minggu! A few of my mum friends said theirs macam on-and-off je, worse when tired or dehydrated, so try minum banyak air and rest more. But since it's bothering you and making you anxious, better check with your doctor or midwife just to be sure everything's okay, especially if ada pain or bleeding. First pregnancy memang scary, I get it!