At 18weeks u should be able to differenciate between flutters and gas. Try lying by ur side. When the baby moves u will feel as if someone poked u from inside. Thats how i felt my baby moves for the first time and i knew it instantly because it was different.
What does it feel like? I felt my baby move from 12/13 weeks. Felt like flutters and sure enough my midwife said you can feel from as early as 12 weeks depending on the lady.