I think it sounds OK. She is just remembering you and wanted to let you know. My guess is, it probably will not be a regular thing she does.

My husband went to see a therapist for 4 sessions, about 3 years ago. Then he stopped. A few months letter, he received a brief, handwritten letter from her saying she hoped he was doing well, everything was going OK for him, and she wished him the best. It seemed really nice and not intrusive. There was no further contact.

I guess an alternative possibility is that your T is hurting for clients now, and fishing a bit to see if you would want to resume seeing her. ???