I agree with the above posters. You don't develop patterns in babies movement until they are a little bigger between 25 and 28 weeks. This is my fourth baby and when he first started bouncing around he would move all day then the next day it was like he wasnt there. I knew it was normal but you still worry. Congrats!
by first pregnancy, that means first to make it this far.
Especially if this is your first pregnancy, you aren't likely to feel continuous regular movements until around 24 weeks. What you feel can depend on the babies position, if s/he has changed positions you may not be feeling movements as you were. If you are still concerned, call your physician and ask if you can come in just for some reassurance.
I get your worrying but at 21 weeks you won't get constant movements the baby is still only small and has alot of room to still move about, they don't generally tell you to start kick counts until 28 weeks because then you get your movements all day
If you are really worried tho you could always ask them to listen in to the heartbeat tO ease your mind