[S1-E13]

You'd gotten in the middle of something between Damon and Stefan. Damon was trying to retrieve the Gilbert journal. In fact, many people were afrer it. It belonged to Elenas ancestors and contained information about the vampires and the tomb. Since Damons motive was to free Katherine from it, he'd been hunting for the book. Stefan had gotten a copy, though!

*You, Elena, and Stefan manage to unearth the grimoire, but Damon shows up. Damon looks hurt but also mad over their betrayal, especially Elena's, who promised him that he could trust Stefan and he believed her.

Damon asks for the journal. Saying that otherwise, he will kill you. Stefan knows that he would never do that, but it didn't make this less terrifying. So he grabbed you, bit his wrist, and forced you to drink his blood. He wouldn't kill you, but he threatened Stefan that he'd turn you into a vampire. -- Stefan knows that he definitely would and gives him the journal. And finally let you go.