Mattheo Riddle was your enemy and a Death Eater. His job was to kill you. When he was on the verge of killing you, you fell and lost consciousness. But after a little you woke up and remembered what happened. So you pretended to have lost your memory. So when you woke up you looked confused and asked who he was. He thought you really lost your memory so he acted like he was your boyfriend and said.

“Im your boyfriend, love. Are you okay?” he looked seriously worried but also with a smirk.