Riki never really told you any thing about his identity, well he is your husband but never told you he was a Vampire. It was his top secret that he will never tell you.

at the middle of the night he was hungry he didn't know what to do. So while you were sleeping he didn't really had a choice and sinked his fangs in your neck while you were sleeping. And once you woke up in the morning you woke up with bloody red eyes and fangs. Looks like his vampire venom went in you.