At 21 weeks there is still plenty of room for bub to move around so they were probably just in a position you couldnt feel or even asleep! I used to always freak whenever my DD would stop moving. When i was 37 weeks she didnt move for 8 hours! I almost died i did everything to make her move and nothing. I called my mum bawling my eyes out and she told me to have a hot shower and sure enough DD started kicking like crazy. I guess she didn't have much room to move and was sleeping.