My girl took a prank personally why is she so mad?

I had just gotten out of the shower and my girlfriend was laying on the bed reading a book. I felt something coming so I went up and whispered 'wanna know a secret?'. She smiled and just before she answered I slammed my bare ******** up against her temple and farted really hard and loud. I laughed hysterically at my own genius and she turned pale white and then the tears started flowing. I tried to comfort her but she was angry and asked why I did it. I told her it was out of love for her but she didn't believe me. I got her a towel for her temple and surrounding area and for the tears. I thought she would understand. It was so just an intimate joke between lovers, shouldn't she be more understanding??