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I'll give you a best case and worst case scenario, okay? The symptoms you are describing have the potential to indicate a serious problem, so I am going to present my thoughts in a straight-forward manner.
First, best case: if your grand-daughter has experienced loss or trauma in her life, some of what she is experiencing may be a delayed reaction to the stress of processing the trauma. In other words, it may be PTSD that is beginning to boil because of the hormonal changes that are inherent in puberty. Counseling to treat the trauma and help her to control her emotions and understand some of the "dissociation" she is feeling will be the best course of action - although it will be a long process, it is reasonably routine to treat.
Now the worst possibility - not probable, but possible: She may also be teetering on the verge of a thought disorder. A disorder like schizophrenia begins when the person is a teenager, although its initial symptoms are not the bizarre ones you think of (like hallucinations and delusions - those crop up in the late teens or early twenties) when you think of a "schizophrenic." Starting in the mid teens, these folks experience what are called the "negative symptoms," which include inappropriate emotionality, strange thoughts, poor hygiene, and social withdrawal. If she has a blood relative who has/had a thought disorder, then the odds get stacked further against her. My fear would be that she may be starting down this road.
The third possibility is that she is being overly-dramatic with you and exaggerating the symptoms. Some emotional fluctuation is normal during the weird middle school years, and hyping the angst to grandma is not unheard of. Add to that some dark ("goth?") thinking, and you have a good tale to tell.
I don't know which of the three possibilities is accurate, but at the very least she is depressed and should be evaluated. During the course of the evaluation, the other issues I mentioned would also be examined.
I really didn't mean to sound dire in my response, but I know you want what is best. If there is a problem, let's get it identified and treated.
I wish you well. If you are satisfied with the response, please hit "Accept." That is the only way I receive credit for my answer. Thanks-