I seem to remember a similar thread recently. This is not uncommon. We have BTDT. After a lot of digging we found one episode was triggered by a fear of dying. DD was thinking if she hadn't been born and didn't exist then she wouldn't be afraid of having to leave, to die. It has happened at other times and she is old enough that we can talk about the search for meaning in life. James Webb has good things to say in his article at SENG.

http://www.sengifted.org/archives/articles/existential-depression-in-gifted-individual

I also have this book by Webb

http://www.greatpotentialpress.com/searching-for-meaning