I feel guilty for laughing so much

So yeah, my parents say I was a big laugher ever since I was little. Today, we were playing a game in PE class and someone fell down. My friends kept saying how she might be super hurt and were all sad for her but I just started thinking about how she fell like a football player. When they all looked at me expecting me to say my part that I'm sorry for her I just started laughing/giggling. My friends thought I was insane to laugh at such a situation and looked disappointed in me. I'm not a bad person though, I genuinely did feel bad for the girl that fell down but I can't control my mouth just randomly laughing at places I don't want to! This has happened before, once when my friend was talking about how her cat almost died because it was choking on a piece of string and when we were watching video in science that I thought was really funny but it wasn't.