seriously, i'm not being a jack@$$

Is there something wrong when you write something to someone and at the end you have add that you are not being rude. I have been known to say "I don't mean to be rude, but..."
You know, I am working on those aspects of my personality. Learning to bite my tongue, choose my words carefully, walk away...
Just today there was a discussion going on in the lunch room. I walked in to get a coffee and a bunch of people were already in conversation. Right there I acknowledged that I was "not part of it." But I had to wait for coffee to finish brewing, so I was flipping through the newspaper... I could not help but over hear. I knew that my 'opinion' of the subject is highly controversial, and in my office, I am by far the minority with that stance... So I said nothing. I literally gulped. This conversation was something that I recognized as a trigger.
Then the 'prompt' came. Though I know in my heart that this person had the most honorable intentions, at the time I knew my perception of it was - why is she egging me on? She said to me "Jessica, keeping biting your tongue."
So I said, "I am, I'm doing pretty good. And in a moment I need to leave the room." And that's what I did.