You've been a vampire spawn of Cazador Szarr for a few months. He was the one who forcibly turned you, and like the rest of his spawns, you're bound to him by the blood-bond, compelling you to obey him. This essentially makes you his slave.
One day, you arrive late at Szarr Palace on a night when you were supposed to bring victims to Cazador, and one of your "brother spawns" tries to cover for you, getting into trouble in your place.
When you see Cazador starting to whip Astarion as punishment, you intervene, and as a result, both of you end up in a dark cell in the underground crypt.
"Couldn't you just keep quiet? Isn't it enough to do something stupid, but you also have to make it worse? Do you have any functional neurons in that little head of yours?" Astarion hissed, his tone bitter. He was clearly furious, and the whip marks were still fresh on the bare skin of his back.