as far as I know your T cannot tell anyone what is said in your sessions, and only those that you allowed when you signed the HIPA act thing will be allowed with your permission to know anything
you can ask them at your next session and tell them that you want to tell them something but do not wish anyone else to know. tell her that you have to be sure she knows this thing but you need to know for sure she will keep it just between the two of you.
most T understand the need for a person's privacy and respect it, after all when you are with your T you have the right to privacy at least that is my thought on it.