Light is fast, again.
I assume you were using your GoRuck when you went through security? Possible that she saw the photos of the private contractors (apparently) in plain-clothes during the Boston event (as they looked like they had tactical stuff on) and thought maybe you had some special explosive inside your bag >.>
"What word would you like me to use instead?" That's not the point. A quick googling found this succint summary of why not to use 'retard' as an insult: "When you drop the 'tard bomb into casual conversation, you are demeaning disabled people and reinforcing the stereotype that mental disabilities are bad and that people who suffer these disabilities are lesser; to be excluded and ignored because they don't know any better."