I am pretty sure what the OP is describing is specifically called emotional lability. In a neurological context is it psuedobublar [eh, spelling?] affect.

It can be caused by many things- TBI or other neurological complications or problems, extreme stress, exhaustion. It can be caused by some medications as a temporary or long term side effect or have roots in dissociation or, as was mentioned- psychosis.

Because the causes are so varied, it is difficult to pinpoint a cause. If it is an occasional thing it is likely not something to be terribly worried about, imo- but I mean, not a professional here... I just have personal experience with this after a TBI. The occurrence of inappropriate affect has lessened in intensity and frequency since the injury but still happens sometimes. Occasinally still with great severity- particularly with sharp unexpected pain- my immediate reaction is to laugh. I have a pretty immediate laugh reaction to a lot of things but it has lessened greatly in general.

I get giddy at some things that should make me feel just the opposite. The more I am aware of these inappropriate reactions the better I am at regulating them though.

If it is happening frequently and with in tensity and most of all interrupting daily functioning you should talk to a provider about it.

If it is bothering you in general, I would bring it up for discussion.