While I don't think this is cause for worry just yet, you certainly can mention it to the doc. Unless you've got other issues though, they probably won't do much. Most you could get is a doppler check or an ultrasound.
Zoë has had her quiet times, where she's not quite as active as she may have been the day before. We're at just over 22 weeks though and more often than not I feel her several times throughout the day lately, probably the past two weeks or so. I think baby just needs to get a bit bigger. If you get too worried, you could always chug some juice and help wake her up. Zoë is crazy active after I eat or drink something sugary.
As has been said, won't be long until baby is moving so much that you won't be able to sleep or get comfortable. :)
It could just be that baby flipped around, meaning maybe the feet were up and kicking you before, and now baby may have his/her back or head up and is kicking down below where it would be harder for you to feel something.
Megan, my pregnancies have all varied in how the baby moves.