Hmm. You're interpretation is fairly accurate, but maybe it was one of three things.
1. Maybe your coordinator didn't like your cocksure response. You have told her that you are quite secure in your position. Perhaps a bit more polite response would have been, "I'm learning quickly and really enjoying my job."
2. Maybe your coordinator was teasing you, and taking a literal translation, suggesting that if you just sit on your pretty *** all day, you aren't doing your job.
3. Maybe your coordintor didn't understand the idiom.