With my daughter, I wasn't sure if I was feeling anything and tried to convince myself I was around 15 weeks. Maybe it was flutters, but to this day I'm not sure if it was just indigestion! At 21 weeks at our ultrasound, I was lying on the table waiting for the doc to come in after the tech left the room and my belly jumped! She kicked me like a soccer player and from then on, it was non-stop! No more wondering!
At 15 weeks, they're still pretty small, so a simple change in position can change what you feel. That said, if you had been feeling regular kicks and suddenly they stop, it's best to see your healthcare provider.