I agree with Sabata somewhat.
She was supposed to have died, and the Thirst became too strong for her to endure. So, in True, she was 'killed' by Roa and disappeared. But as Shiki killed him, the energy he stole for Arcueid flowed back to her and thus she returned. However, in this state, she's largely fragile, and even though she rested a lot, since she already had the taste of blood, she's afraid that her urge for blood eventually hurts Shiki, so she goes back to her Castle to sleep. Of course, this in the True Ending.
On the Good, it's mainly for the Arc-lovers out there (like me) who can't support a sad ending. She got all the energy from Roa, rested a bit, and being a True Ancestor and all, resisted the Thirst to stay with everyone's favorite hero.
The End, enter KT & MB.
Hope that clears up something