Aiyo, jangan worry sangat lah, at 23 weeks, memang kadang gerakan baby boleh kurang terasa, especially kalau placenta dia in front (anterior placenta) or kalau perut kita memang thicker, macam nurse cakap tu. A few of my mum friends also experienced the same, habis takut jugak, tapi baby mereka semua fine je. The hardening you feel is probably Braxton-Hicks contractions, which is totally normal at this stage, tapi better you mention this to your doctor at the next visit, especially since you expected to feel more movements by now, they can check baby's position and heartbeat, terus you boleh relax.