Hmm, at 23 weeks tu memang perut boleh keras on and off, biasanya dari Braxton Hicks contractions yang normal dalam pregnancy, especially bila dah mid-trimester. But kalau keras tiba-tiba or sama dengan masuk angin symptoms macam perut kembung, cirit-birit, maybe dua-dua la, so best check dengan doctor just to make sure everything okay lah, a few of us had the same thing and the doc confirmed it's normal tapi better safe than sorry kan.