@Riger and from the fact that there is only one (and it's pretty deep), looks like she never let whatever caused it happen a second time.
Based on her reaction to the mage's words, I'm guessing it was a betrayal. The traitor might have even mocked her when she asked why, by either saying she's "just a tool for war" or they made fun of her lack of femininity. Either way, the words would hurt, and she'd remember them whenever she feels or sees the scar.
I'm probably wrong, but this is just a wild guess based on the first 2 chapters.