Its normal hapoens to me all the time when in the night because when theres a akward silence ill burst out laughin "ps. Im a night person" and my parents will shout shut up so ill put some music on till I pass out I cant keep quiet at night lol
It depends, does she laugh by herself several times a day and if so is it for elongated periods of time? Also does she appear to be having a conversation with herself while laughing? Can you make her stop some how? If not it is very possible that there is something wrong and you should be concerned. I have experience with this problem, not me but someone very close.