Is she on some kind of medication? sometimes those can cause changes of behavior and the person doesn't even realize its happening I mean that just seems like a pretty major change which is why I ask...the way you describe it does make it sound fake....I doubt she is really 'happy'. Or maybe its some sort of psychological problem just doesn't sound like shes all that stable but is acting like she is. I really don't know what to do though aside from try to have a serious discussion with her about it...but that could also go terribly so I don't know.

How long has she been that way? Not sure quite why you posted it here...perhaps you are thinking she might be traumatized by the death of your father. SO what I would say about that is sometimes its not uncommon for people with PTSD to try and ignore it and suppress it to try to just move on, I've tried that though I don't think I went to an extreme of acting really happy all the time just better than i felt around people...so it could be something like that but I am certianly in no position to diagnose let alone do so over the interenet.