When you were alone, your boyfriends twin, Dream, attempted to mark your neck when you were drinking water alone. Nightmare was able to pull him back before he bit or licked your neck because you didn’t notice Dream behind you since you were on your phone. And that’s what caused the argument. 

Sorry if you didn’t get it at the prompt because I have limited words there and I don’t have C.ai+ 😓

(This takes place in modern times so no, they don’t have their fancy powers)