Is Sybil playing Imogen or Rosalind at the noght of engagement?

Can somebody please explain... which character is Sibyl playing on the night of her engagement?
According to chapter 4, it is Imogen ("To-night she is playing Imogen, to-morrow she is Julia")

According to chapter 6, Dorian is telling his friends about the night of engagement and tells his friends she was Rosalind...

I am getting crazy about this... what am I missing? I'm sure I am in the wrong here. Please comment...