Hi Omers,
That really sucks. Usually it is the client who lets something out right at the end. It is called "doorknob therapy" as elliemay said. Usually a client will be walking out the door and add something really important or significant.

My only guess is that she ended up speaking something that was meant to be an internal thought. IE she was noticing that you were still uncomfortable in session, and ended up speaking it out loud to herself rather than just thinking it. So she didn't realize that she said it as she thought it was an internal thought. Ts are supposed to have things called 'silent hypothesis' about what is going on with their client. These are hunches that they are then supposed to see if they are accurate or not through the more information they get from their client. But they aren't supposed to bring them up with their client until they think their client is ready to talk about it. For example, if a client is abused as a child but denies having any negative feelings about their abuser. The therapist might think that there are some negative feelings about the abuser, but the client is not ready to talk about them. So the therapist would notice this, and then let it sit until they thought that the client was ready to acknowledge any negative feelings that are there. This is just an abstract example.

I hope you bring this up next session with your T.