Hi Jordy,
I think that because, as you said, it was out of character for you to do that, your T was just trying to confirm that that things were okay. It could have been a termination letter or a suicide note, or who knows what else. She did the right thing (or at least something my own T would do) by confirming that you were okay.
Also, congrats on allowing yourself to be forward and vulnerable enough to present her with such an open letter. Those things help the T see you best, and in turn helps them help you better. I'm willing to bet she already read the letter but is waiting until the session to address it since, like you said, the topic is able to hold out until then. I have no doubt that she cares but I am not so sure that she will mention the letter on Monday unless you address her to ask her if she read it or something.
I think its normal to question if you made the right choice but I think its very helpful that you chose to be bold and do it and I hope it all works out for the best for you