I think that is probably the baby. Most women start feeling something around this time.

Quickening (first time you feel your baby move) may start as early as 15 weeks esp if this is not your first pregnancy. But on the average, esp if this is your first, you should feel your baby between 18 - 24 weeks gestation. I felt mine between 15th & 16th week and it feels like a bubble & gas. Few more weeks my dear and your baby will do more movements that will definitely bring you a lot of smile and soon a bit of discomfort bec your baby choose to be more active when you are sleeping! Lol

It's probably the baby. However, more frequent movements for me didn't start until about 19 weeks.