There were probably a few things going on, at least from what I can see.
You set a boundary (limit) by standing up to her and letter her know you can't help her. You used validation. And you formatted this whole thing in a shorted version of the communication techinque we call SET - Support, Empathy and Truth (you omitted the "S".
We often teach others how we will tolerate being treated. Its good for ourselves, and others, to know our limits. You've started the process of commnicating some of your limits ("I can't help you", which is also the T in SET).
You also used validation nicely to relate to her feeling a lack of trust - she was probably also feeling a lot of frustration to begin with (unable to remember where she left her cigarettes).
Keep reading and keep working. I remember the first time validation worked for me. We learn a lot of this by trial and error, so keep at it!