I second everything that farmergirl said. The word is triggering for you because your boyfriend used it in an abusive way, as part of the abuse. But it's a valid word for the T to use and she's not being abusive.

But there's absolutely nothing wrong with you getting angry at her for using it. It makes perfect sense, it's valid, and you should tell her. If it was me, I would say "I HATE it when you use that word!!!!!" Then we would discuss why. If you'd like her to remove it from her vocabulary when she's in session with you, ask. But the important thing is to work on the issue it brings up.