Baby moving

Si

21 weeks, and baby was soooo active last Wednesday, Thursday and Friday for the first time. I’d only felt little kicks here and there before. Saturday and Sunday she didn’t move much ): if I poke her she wiggles and gives a kick. But not much of her own movements. Found her on the doppler everyday and she’s still in there happy but just sleeping!! Why do they go through active phases and then slow down?? I haven’t felt her at all this morning and I’m not worried. Just missing my kicks!!!