Movement at 19 wks

Is it normal to feel lots of movement one day and then much less the next? Is baby just tired?

I had a really long day yesterday, between clinicals at school and an 8 hour shift on my feet at work, but I felt lots of flutters in my belly. I'd been feeling them pretty much daily for a few days prior, too. Today there's hardly anything at all. I don't want to worry, but I was getting used to feeling baby's little tickles. Could he/she just be tired from being so active yesterday?