One way of looking at it is she has effectively changed a boundary, and that is always hurtful and confusing to deal with. If she didn't want clients viewing her Facebook she really should have locked it down from the start.
It makes sense to me that you feel hurt and ticked off - I would feel the same. Sharing that you had been viewing her FB in therapy was the mature thing to do - so well done for that. I think your T has been a little bit careless with your feelings, and I hope you can talk to her about this.