The exact same thing used to happen to me until I was around mid-teens and I was extremely embarrassed about it. Again always when friends made me laugh and usually just a bit, but occasionally I lost bladder control completely which was mortifyingly embarrassing. My mum never mentioned it as far as I remember, and I did eventually grow out of it (doesn't happen now even after 2 dcs)
I wonder now if it's a hereditary bladder thing, my sister has also had bladder problems since a child, although hers were diagnosed by a dr.
So is this common or do I need to get her checked out? She was a late bedwetter too, not dry till 7 at night.