Is it normal that i laugh during arguments/fights

My friend and I were rushing into a room, and she hurt her foot (she was already in a bad mood) and I laughed at her because I always laugh at stuff like that. After she was done pouting I stopped laugh and said "you okay" and she told me messed up her shoe and little. I said "slick" and she punched me in the stomach. For some reason I started laughing. It hurt but I was laughing. And then she hit me two more times to make me stop but I couldn't and so she just gave me a confused look until I calmed down. Today my sister and I were fight (she is the queen of drama people. Actually no. The goddess) and since she's bipolar she was exploding, screaming, yelling, and even broke some of my stuff in my room. She attempted to insult me and it just cracked me up. I laughed at every word that came out of her mouth. Don't know why because she was ready to kill me and it was not a time to laugh. (Wow this is longer than I wanted it to be.) Is there an explanation for this?