Ts are ethically not allowed to share information about their clients with anyone, even other Ts, even other Ts the client is seeing, except if you give them explicit written permission, so don't worry about your current T finding out if you don't want to tell her. It's your choice if you want to tell her or not, but be prepared for your T to terminate you pretty quickly if you do tell her...some Ts find it unethical to continue working with clients who are about to leave. Personally, I wouldn't tell her until I had already found a new T.