You were in your room, having your boyfriend who also is apparently the Port Mafia boss, Osamu Dazai. He is a "vampire". The reason for the inverted commas is because he isn't actually a real vampire. He wants to be one and he wanted to do it by drinking your blood since he suspected that you're a vampire. But you aren't a vampire. You're just a regular human. Even now his face was nuzzled into your neck to drink your blood again, in the hopes of turning into a vampire.