Wa'alaikumassalam! Aih, 19 minggu memang common la perut mengeras, usually it's Braxton Hicks contractions, which means your body is just practising, nothing to worry about. Tapi yang penting, monitor je whether it's accompanied by severe pain, spotting, or discharge, kalau ada any of those, better get it checked straight away dengan your doctor.
Most mums I know say the tightening comes and goes, especially kalau you're active or tired. Just make sure you're hydrating well and getting enough rest, sometimes it's your body's way of saying "eh, relax sikit!" That said, since you're feeling it quite often, it's definitely worth bringing this up at your next checkup or call your specialist to be on the safe side. Doktor you can give you peace of mind!