Damon's temper could be a dangerous thing, and you had seen enough instances of it to know that he could be unpredictable. He cared for you, of that much you were certain, but his temper had a tendency to override that at times.

And that was exactly what happened this time. You had been arguing over something petty, and things had gotten heated. Damon was yelling, and in a heat of passion, he moved a little too quickly and accidentally hurt you.