The line between assertiveness and criticism

Hi Brooke! I wanted to say that I struggle A LOT with communication at work. I find it so difficult to approach someone and provide what I think is constructive feedback or constructive criticism. Your podcast on Confident Communication helped me A LOT, but I currently have a “thought loop” on a scenario that happened to me recently. Before I listened to your podcast, I was at work and was found in a situation where the secretary was trying to call me in the medication room (I am a nurse). I was unable to pick up the phone in time, so I called her back and asked her what she needed; after our conversation, she said at the end: “Next time could you pick up the phone so I don’t have to be looking for you?” with a tone of voice that I perceived as aggressive & critical. I told her I would. But the conversation kept bothering me later in the day, so I approached her about it and said, “I felt hurt by the way you said ____… It came off as a little offensive.” She was confused because, in her view, she had done nothing wrong and was upset that I had told her that. In my mind I was trying to be assertive to let her know how I felt. But after listening to your communication podcast, I think I just heard her message wrong and interpreted what she said incorrectly. If I put this into the model:

C – Phone conversation with secretary who spoke with loud tone of voice, saying, “Next time could you pick up the phone so I don’t have to be looking for you?”
T – I thought, “She is talking to me in a mean way. She has no right to talk to me like that.”
F – Angry, upset (I felt like I was being criticized)
A – I talk to her about it, feeling upset and hurt.
R – Secretary felt upset/confused because she did not think she did anything wrong, we both did not feel that the issue was completely resolved.

I thought about how I could have approached that better. Maybe I could have asked her to clarify, instead of flat out saying how I felt. I could have asked, “Hi ____, I know we talked in the medication room, but I just wanted to clarify with you what you were asking me because I want to make sure we’re on the same page.”

Would that have been a better question to ask her? Or should I not have confronted her at all about it (because I know I personalized what she said and made it mean something negative)? What is the line between letting someone know they upset you, versus letting a comment go and allowing a person to behave a certain way because that is how they talk/act (knowing their behavior has nothing to do with me)?