mentally disturbed

I used this term "mentally disturbed" in my complaint letter to the head librarian. I wonder what does this term "mentally disturbed" mean to the native English only speaking ears. You see, I borrowed this one book from a library, and I returned it before its due date. Then later, I got an overdue notice saying that I still possessed an outstanding library book. So I went down to the library, showed one of the clerks the overdue notice, and explained to the person that I already turned in the book and that I didn't have that book anymore. So that person asked another helper to locate the book on the bookshelf. I waited and then the helper showed up with the book that I borrowed. They talked among themselves about what to do. Then they said that I still owed them the fine for the book. They insisted that it was possible that I put the "overdue" book back on the shelf myself. I still maintained that I did not shelf that book and that I returned the book some weeks ago. They said if I wanted to dispute the fine, I should write to the head librarian. Since I had a headache and was emotionally annoyed at the time of quarrel, I decided to use the term "mentally disturbed" in this context "I was mentally disturbed when they accused me that I shelved the book myself."

To me, "mentally disturbed" means you have some kind of emotional abnormality or mental illness, schizophrenia and the like!

You could say "I was upset" or even "emotionally perturbed" (although that's kind of unusual in non-professional contexts 👁 Wink ;)
).

Thank you, Cropje Jnr. So I should have used the word "upset" instead. It is such an easy word, and I couldn't even think about it. I was trying to sound formal in the letter. Little did I know that I was describing myself as a crazy person in the letter. Now I learn my lesson.

Yes, Cropje Jnr is right - "mentally disturbed" is not what you mean here. It would be the plea to say that you admit you were wrong but were mentally ill at the time.