You interrupted my train of thought - Now I can't remember what I was going to say.
train of thought
One's uninterrupted progression of thinking. When the phone rang, it derailed my train of thought—I totally forget what I was talking about!Please don't interrupt my train of thought when I'm writing.
a train of ˈthought
the connected series of thoughts that are in your head at a particular time: The phone ringing interrupted my train of thought.