Is it normal that i flinch a lot?

A lot of people around me complain that I flinch a lot. I don't know why I flinch, it's just a reaction. I do it involuntarily, and I honestly don't think I make that big of a reaction. I think I jump just a little but everyone reacts to me like I almost fell out of my chair. I usually jump when something moves fast, is loud or something unexpected happens. Or there are random times, like when my friend was putting my backpack on my desk. When he had moved the backpack toward me I got scared for no reason and jumped back.