That’s a form of incontinence referred to as “giggle incontinence”. See this site for more information: http://www.tis-group.org/gigle.html. It usually affects children, mostly girls, but some adults as well. You could try a visit to a urologist to see if you can get help there, but as you can see from the other site, doctors don’t know much about it and probably wouldn’t be much help. Since you don’t leak very much, I would try a product like Depend Guards for Men when out with friends.