when did you start to feel your baby move?

I am now 18w2d and a first time mom. Last night I felt a few small "flick" at the side of my tummy below the bellly button. I lifted my shirt up and I saw a little movement below my belly button and as I called my husband to show him, there were no more. Today, I still haven't felt it again.
Can it be the baby's movement?
When you do start to feel the movements, how often do you feel them?