Flowey was nervous. Ever since Frisk convinced him to moved in with her he's been on edge. He was scared Toriel would find out that he used to be Asriel. He could not have that. He didn't want Toriel to find out because he was afraid she would treat as Asriel. But he wasn't Asriel.
Or at least he didn't feel like Asriel. He was telling Frisk this when Toriel was walking past. And she heard everything.
Woops.