Maybe she feels bad about dumping her ex and now wants to reconsider the past and find out if she still has feelings for the past. I bet she cares little about how much this is hurting you, but still it is better that she is honest with you than doing it behind your back.

The worst situation i can think of is complete freedom for her to keep on going back and forth between you and him and not wanting to make up her mind. Maybe some distance is needed here for her to make up her mind once and for all...

I find that, in a relationship you can only have feelings for one. Once more people are thrown into the mix the relationship begins a race towards the bottom.

__________________
My Demon revokes any prayer
He's grown contempt for love and hope
He betrays trust, twists truth and fair
Indifference is his way to cope
Engulfing sound of sensations
He quells with voices of despair
And muse of short lived inspirations
Flees at the sight of his cold stare
~Moy Demon - Mihayeel Lermontov~