I sometimes get bursts of uncontrollable laughter like this. Like one time I was on a bus and I just started laughing my head off. It was silent on the bus (apart from my laughter) and I could see that the girl I was sat next to (who I didnt know) was really uncomfortable. But the fact that I was laughing for no reason on a bus was funny, so I laughed even more. Even after the journey ended, I carried on remembering it all day, so I burst out laughing in shops and the like, and even when I was in bed at night, it kept happening.