i encountered this 2 wks ago when i was at my 28th week. baby suddenly wasnt as active as normal, emailed my gynae, she said this was quite normal but for a peace of mind, i still headed down to the hospital for some checks. luckily baby is all good. it could be due to baby's position as well, so his/her movements may not be as obvious as usual. but please do monitor and check in with ur gynae.
Jasmine