Depending on how long ago the previous relationship ended for her, perhaps there's still feelings for the other person or unresolved issues both personally and emotionally.
If she is just getting off the drugs, perhaps it's too soon for her and/or you to be committed to each other. Maybe she needs to work on herself first before trying to be there for someone else.
But also in all fairness to you, I would think that you would like to start a relationship that is open and honest too, even when it comes to past relationships, but even past ones have their moments of privacy to the person directly involved and it's just something that she may not be ready to share with you.
Good luck and I hope I somewhat helped or answered your questions...