@Pazzie If you go into the apartment's entrance area where the elevator is located, you should see two (or three, if there's another apartment on your floor) objects on the walls.
One will be the mailbox. The other will be the trash chute.
If your sim has been bitten by a vampire, he has been "turned."
His stomach should begin occasionally glowing (like after an alien abduction, but purple swirls instead).
In about 48 hours, he'll transform into a vampire. If you don't want him to be a vampire, you'lll need to max Vampire Lore (Level 15), then use a mixology bar to mix the Ultimate Vampire Cure.
It costs $2,500 and requires your sim to have 10 plasmafruit, 10 garlic, and 10 wolfsbane in their inventory.
My condolences if you didn't want your sim to become a vampire.