Baby not moving 21w

Someone tell me why my son won’t move all day but during the night he moves like crazy. It’s to the point I’m about ready to go to the hospital bc he doesn’t move from the moment I wake up until right before bed. Right before bed he’ll move and kick. I never experience this before with my girls once they started to move they never stopped. I’m 21 weeks now. I started feeling baby move at 16 weeks. (My first i felt at 17w and my second at 13w.) is my son okay or is this normal?