Why does my 2 1/2 month old scare so easily?

My son is almost 3 months old and he scares extremely easily. I also have a 3 year old and she was never like that as an infant. My son will hold his fists so tight to his chest and scream like ah.. ah.. ah.. ah.. type of cry with his eye extremely wide. When I got to change his clothes or sometimes even change his diaper he will do this. There are times where he won't sit in his bouncer or definitely the swing he will sit there and scream. I don't understand why he is more scared than most babies get at this age. I know things are still fairly new to my baby but it seems everything is more scary to him than it should be. Can anyone help me understand?