I'm glad you've been feeling better,
Trog. It sounds like you've been keeping yourself busy.

There could be several possible reasons for your ex's change.
She says that she's depressed. Borderlines tend to struggle a lot with negative emotions, and depression (or depressive episodes) isn't uncommon. Some medications can cause mood changes, too. I struggle with depression myself, and I know that I'm withdrawn and not overly communicative during bad times.
It could be that she's in a more detached-protector mode now with you. She doesn't want to get into potentially emotional discussions, so she's trying to avoid that territory.
It could be that this is her "real" persona - just her with no mirroring, and no love-bombing, etc. Maybe she's just a bland, disinterested person.
It could be a game. It could be an attempt to get you to feel pity for her and try to draw her back out. It could be her way of "punishing" you for "calling her out." It could be that she's purposely acting boring to push you away.
There are many possibilities, and they're not mutually exclusive.
The important thing is how this makes you feel. You say that this contact has made you feel better and helped you in your healing, which is a good thing. Don't feel bad about your thoughts.
You say you're worried that she's "playing you on a game of some kind," based on your history with her. There's a quote floating around the Internet that caught my eye recently - "I don't trust words; I even question actions; but I never doubt patterns." Even if this contact is not a game, her patterns indicate that she can't avoid game-playing for long. Is that something you want in your life?
Do you feel like you want to continue contact with her?