IMO, I would take the card at face value and not try to analyze her intentions as it would only be speculation at best.

If I were you, I would write to her and thank her for the birthday card and wishes. Tell her you hope she is doing well. I would keep it short and sweet. This puts the ball back in her court to do with as she sees fit. If you hear back from her, then you know she wants to have contact again....at that point you can open up a dialoge more deeply about your relationship.

I wish you well!

Hugsss
sabby