Every baby is different but they shouldn't be moving less as they get bigger. Just the movements change cos they don't have the same room. I always thought they moved less the further you got but nope every midwife has told me they shouldn't drop their movements. But you'll know what's "odd"for your baby the further you get etc :)
Yeah, more then likely just mean you baby is running out of room to move as much because of how big it's getting. It's pretty normal just means you baby us growing & getting big & getting ready to make it's grand entrance :)