What type of mental illness does Nina suffer from? It looks like she has schizophrenia. Her mother also appears to be unwell or just extremely overbearing.
The production requires a ballerina to play the innocent and fragile White Swan, for which the committed dancer Nina (Portman) is a perfect fit, as well as the dark and sensual Black Swan, which are qualities embodied by the new arrival Lily (Kunis). Nina is overwhelmed by a feeling of immense pressure when she finds herself competing for the part, causing her to lose her tenuous grip on reality and descend into a living nightmare.
So just like @aman_abhishk said, it was pressure.