Is that movement?

Okay, I'm turning the corner into my 18th week and I'm still confused about whether I'm feeling movement or gas.  I have been feeling something since 15 weeks, and it got stronger last week, but it is so inconsistent that it is impossible to tell.  Up until last week it felt like there was a caterpillar in my belly, now it feels more like something bumping around.  I am beginning to get impatient, I want to feel my little munchkin!