She's afraid?

I was eating lunch with my sister and my little step-sister. My step-sister, see, she was sitting next to me. The thing is that I was going to fart and I couldn't barely hold it so I just let it out. She paused for a moment and then stared at me. I imitated her laugh and after awhile, she moved away. My sister asked her if she was afraid of me but she shook her head. When I stood up to finish my drink, she quickly grabbed her plate and went upstairs. Weird -- I didn't know she was afraid of me. :(