I wouldn't be too concerned, some women are lucky and can feel their babies move early. But, most don't feel the baby until 20+ weeks (especially in first pregnancies because you don't know what you are feeling for and it can feel like gas). Babies are still very small at 18 weeks, and really cushioned in amniotic fluids. Your last trimester is when you start counting movements/kicks... and at that point you can just drink some juice and your baby will kick you in the ribs...lol :)
That could b normal.the baby could of moved and u just didnt realized it. With this pregnacy im 15 weeks and some times I feel the baby some times I dont cuz its still a lil early to feel them all the time