Baby is not moving as much

So since week 18-20 I felt the baby moving and kicking like crazy sand felt like every hour of the day. Now it's week 21 and for the past 4 days the baby just isn't as active as before, I still feel her moving but not as strong or as much. Could it be she's sleeping when I'm awake and awake when I'm sleeping? Or behind the placenta so I can't feel her as much? I'm not sure if you guys know anything?