Eh sayang, perut mengeras kat second trimester memang biasa lah, especially kalau baru week 16, uterus mu tengah grow and stretch, so akan rasa tight dan keras tu normal je. A few of my mum friends during their pregnancy complained about the same thing, especially kalau diorang active or tired. Tapi kalau disertai dengan bleeding, severe pain, atau contraction-like feeling yang frequent, kena la check dengan doctor cepat-cepat ye, jangan ambil risiko. Minum air banyak-banyak, rest bila boleh, and try not to stand terlalu lama, biasanya membantu!