"And who did you think--" Rose starts then stops and shakes her head. "I don't wanna know. It's--not important." And it's not. She doesn't imagine it was her so in the end it doesn't matter who he thought he was kissing. Drusilla's words give her a chance to redirect her anger a bit until she can figure out exactly how she feels about this and how angry and hurt she is over the Doctor.

"You rooted 'round in his head without permission? You can't just run 'round doing that to people. People ought to have their minds left alone."