I wrote about this same thing (on WP) a while back. It's not uncommon for me to have an "exaggerated" startle response. Some examples:
a) I am at work, focusing on something requiring lots of precision, and someone comes by and asks me a question
b) I am driving on the freeway and a loud motorcycle passes by my car
c) I am sitting in the therapists’ waiting room, playing on my phone, and suddenly the therapist’s door opens
When I have an exaggerated startle response, usually the following occurs:
- My body jerks/jumps
- My heart starts racing rapidly
- I make a sound indicating I was startled (“aaaah”)
I am not 100% certain why this happens, as it seems intermittent.