I agree that her gesture sounds like a reflexive action in response to being engrossed in the conversation. Generally touching of the chin indicates thoughtfulness, so I would not necessarily concern myself with the minutiae of body language in that context.

That said it's difficult not to over-analyze things as simple as a finger on the underside of the chin when you're expressing vulnerable, intimate details of your life, but it's likely meaningless. When in doubt, address it with your therapist. That's the only way you will get a clear answer.

My therapist has generally open body language. When we are discussing something sensitive she leans in with her elbows on her knees and focuses on me with attentive fondness. I tend to scoot away and pack myself into the corner of the couch when she does that. Though I subconsciously mirrored her when she did this a short time ago. Tricksy therapistses.