You should definitely tell your doctor about any change in movement, but also don't let it scare you to death either. Just let your doctor know and go from there. When your baby goes through growth spurts though, sometimes they will have a couple days of very little movement while they rest more. Also I was surprised that I started not noticing when my baby makes small movements, I am 31 weeks and sometimes my fiance will be kissing my belly and he asks me if I felt that and I'm like...felt what? lol he tells me she moved but I had no idea! And I am very in tune with my body and her movements, but I think I've gotten so used to her mega painful karate kicks that I notice her smaller movements much less now.
Definitely get any reduced movements checked out.
Any reduction of movement needs to be known by your OB or doctor