Totally understandable that you would be upset by your T not being honest straight away Scarlett. It sounds like she was kinda 'caught out' telling a white lie as such and realized it was better to tell the truth but at that point the damage had been done.

As others said it is likely she did this to protect you from the feelings or perhaps protect herself from having to deal with that or maybe both or something else. Either way you are still hurting and the trust has been shaken.

If it was my T it would make me question what else did they were willing to lie about to save face, save my feelings or protect themselves and I like you value honesty and openness over anything. It is a seemingly small thing to many but I can understand why it would cause a big rupture. It will take time and talking about it and your T being willing to accept she made an error and being willing to talk about it to work through it. It might also be helpful to focus on the fact that she is human and made an judgement error in the moment.