Baby kicks or gas bubbles ?

Hi I am 18 weeks pregnant today , I felt gas bubble like thing in the past week for two days only then I felt something different than gas bubble . I am not able to distinguish if it's a muscle pull or baby movements ? I am worried for now as I am not able to get the difference . Also it is normal to feel movement one day and not for some other days ? I am trying my level best to calm down but then it's my first pregnancy so I am worried for one thing or the other ! How should I go on about this please ?