Your reflexes have made a bad choice. You end up involuntarily biting Fionna's arm. When blood fills your mouth and you realize what you've done, you pull back.
Fionna stares at you in all out shock. And even though it seemed impossible the fawn started shaking even more.
You run off, leaving Fionna and the fawn alone. When you are far enough away, you realize this is what you should do. Run away...you hurt your friend. But first you need to make a decision.