I have had this for years, most often when I'm tired, where I cycle between wetting myself type laughter and full blown ugly crying. The laughing lasts like 5-10 seconds, followed by crying for 5-10 seconds and the entire episode tends to last 10-30 minutes depending on how quick I can calm myself.

When I was diagnosed the psychiatrist said it was emotional cycling but I haven't found it mentioned anywhere or heard of anyone else who does this.