In a car, i was looking out the window, when suddenly we passed over a bump and my head hit the back of the seat hardly, while still turned facing the window. this caused a snap sound in my head and it aches .note my posture is bad?

A whiplash strain of neck muscles is likely. Hopefully symptoms will be mild. If neck discomfort & stiffness worsen over the next day or so.treatment is applying alternating cold&hot applications, taking anti-inflammatory meds like ibuprofen, wearing a soft neck collar as needed. Your MD is best to examine and advise you if there's significant neck discomfort.