I remember this happening alot with my first. From when I first felt her moving (about 17 weeks or so) until probably 26-27 weeks there were whole days where I didn't feel much, if anything.
Baby spends 90% of her time asleep, and also as someone said may have shifted positions. I think it's fine and nothing to worry about (but I know it scares you).
Eat something sweet and lie down for a while, you will most likely feel something, if not give them a call..