Happened to me in college in a class couple times, I was tired and it was late and something just made me laugh. And I could not stop and the harder I tried the worse it got and kept worrying somebody would be offended or angry but only a couple of people looked at me but I was tearing up literally and turned red and still can't remember what had made me laugh in either case.

But I think your case is different, having happened right after emotional pain. Maybe it was a defensive thing for you to distant yourself from the pain. Sometimes people laugh when faced with deep shame but obviously anything can trigger it.