i thought it was an allegory for Nina coming of age and becoming a women... the tension with her mother, the bodily transformation the sexual awakening, etc...

You seem to be asking two questions. In terms of what the movie is about thematically, I’d say it’s about the immense pressures of show business, particularly the pressures on young women. In this movie Nina is being torn in numerous directions. She has to be an absolutely perfect ballerina, and she also has to let loose. The contradictory and impossible demands the instructor makes on her, his sexual assault, her mother’s overbearing and borderline abusive nature, her rivalry with the other dancer, her knowledge of what happened to the previous dancer in her role (Winona Ryder’s character)—all of it adds up to a complete overload in which her grasp on reality and very identity comes apart.

As for what disorder she has, I’m not a psychiatrist but the following article is helpful:

http://abcnews.go.com/Health/Movies/black-swan-psychiatrists-diagnose-natalie-portmans-portrayal-psychosis/story?id=12436873

Some years back I took a course on movies about mental illness, and one thing that became clear was that Hollywood takes pretty massive liberties with the subject. Black Swan is in a tradition of films that depict schizophrenia as being sort of like a bad LSD trip. (A Beautiful Mind has elements of that, though it sticks a bit closer to the truth.) I don’t go to movies expecting an accurate depiction of these matters.