If ever you feel like the baby's not moving enough, have something cold to drink (or if you have something specific that usually makes the baby move, have that), lay down on your side, and count the movements. If you don't get somewhere around 10 in an hour, try it again. If you don't get 10 again, then call your doctor or midwife. This seems to be the most common advice out there for reduced movements.
I've heard that babies run out of room and move less the further along you are, and I've also heard that that is total rubbish - so I would maybe talk to your doctor or midwife about it and see what they say. I live by the rule of "less movements = bad" regardless of how far along I am, just because I'm paranoid and not willing to take risks.
I've been in for monitoring twice...it kind of sucks, because I felt like an idiot both times since it turned out to be fine...but I'm glad I did it. Peace of mind is wonderful.
I'm sure everything is probably just fine with your baby...but if you're worried, you should definitely go get it checked out.
ETA: There's a sticky thread in third tri about reduced movements....if you haven't gone through it, you should. Warning, it's pretty scary, but it's valuable information.