Wa'alaikumsalam! At 18 weeks the movements boleh memang subtle lagi especially if first time rasanya, tapi since ni third pregnancy jarak jauh, hormones and uterus muscles dah stretched before jadi sometimes harder to notice yang awal-awal. The tightening sensation boleh jadi Braxton Hicks or just the uterus expanding, yang normal je, but if you're worried or it feels different from your first two pregnancies, better check with your doctor to be safe, especially since gap yang lama. A few of my mum friends said they didn't really feel consistent movements until 20-22 weeks with third baby, so don't panic yet! Just keep track of any changes and let your doctor know at your next checkup.