She could just be having a lazy day, but if you're worried it doesn't hurt to call.
My son has lazy days every so often and they freak me out, lol. I have a home doppler though, so it's nice to be able to check heartbeat whenever I want.
If youve tried multiple times throughout the past couple hours and still no movement, id go in. Even on a lazy day you should be feeling some kind of movement at 28 weeks.