baby movement

i'm 18 weeks 2 days and about a week about for a few days i was feeling a lot of movement. but for about 4 days not i haven't really felt anything. something i feel a flutter. it it's so light and fast i can't tell if it's the baby or digestion. is it too early for me to be worrying about this?? i have an ultrasound in a week. when do we start concerning ourselves when baby loves every day??