Not the asshole

Hi, so my girlfriend and i watched a horror movie recently, called The Last Exorcism. Now keep in mind, i HATE horror, i get very scared easily. but she likes horror, and practically begged me to watch it with her. So, i did watch it, and have been very jumpy lately. It was pretty scary, because horror movies have always given me nightmares.

My girlfriend, however, thinks it's funny how i am. She has picked up me bad habit. She likes to sneak up behind me and make a loud noise to startle me. It's horrible. I hate it. I feel like i'm walking on eggshells in my own house.

So, last night, she went out with some friends. I was home alone, and after a few hours, she came back, but didn't tell me or knock or anything. I had no idea she was home. I went into my kitchen, keep in mind, this was after midnight. And she ran up behind me, wearing a scary mask with a scream. Without thinking, out of reflex. I hit her. I punched her in the nose.

She was crying and her nose was bleeding. I repeatedly apoligized to her, but she pushed me away and called me a piece of shit. She packed up her clothes and left my house. She refuses to return my calls and has been telling our friends that i hit her.

I didn't mean to do it, but it just happened.

So, am i the asshole for punching my girlfriend as a reflex for her startling me?