I don’t think you are being too sensitive in reacting to racial slurs. I do think it’s possible that your therapist has seen other indications that you are over-reactive to what people say and that she wanted you to explore the issue further. Both things can be true.
I’m glad you want to pursue working through the issue. The person you should be asking about this is your therapist. It may be that your letter is an example of what she was talking about. Rather than talk with her about your concern about her statement, you have been troubled about it for some time and turned to me for an explanation. Your therapist is the one who knows enough about you to help you see a pattern if one exists. I only have your brief letter. Please talk with her.
I wish you well.