It is Not Depression
every time someone presents like this.
If it has been long standing, then may be its her personality
If it has been a recent change it could be a reaction to any transition/change (in work/college/family/relations/other social situations) or it could be because of any major stress
that she can not talk to you about or is finding difficult to talk to you about.
And finally, yes, it could be anything like psychosis
or depression or many other problems related to psychiatry.
Do try and take her into confidence gently and if she opens up that's ok.
If that does not help, gently encourage her to get professional help, if necessary assuring her that you would respect her wish that this needs to be confidential
Finally you may take her to a psychiatrist
if all above fail.