Gordon had said in interviews about writing this song that it was his imagination, fueled by jealousy, running away on him. He was 'locked down' at home writing for his next album, and his girlfriend was going out every night partying. Note that he says things like "I can see her lying there..." and "I can picture every move..." - it's his imagining what might be happening. Also, i have read speculation that 'Sundown' refers to a specific man he thought she might be seeing, as another singer in southern Ontario at the time this song was written had this nickname...
This was one of Gord's biggest hits and it's primarily about a relationship he had with a girlfriend named Cathy Smith. She was also a backup singer on his records. Cathy led a troubled life and actually spent 15 months in jail for administering a lethal drug overdose to comedian John Belushi. After she died Mr. Lightfoot said of her: "Cathy was a great lady, men were drawn to her, and she used to make me jealous. But I don’t have a bad thing to say about her." Thanks for the excellent reaction, I can see you make every effort to understand what Lightfoot is trying to convey and you're correct, he's a very deep and thoughtful writer!
Hi Cynthia! This song was written by him because of a girlfriend who was being unfaithful. It's been a long time, it seems. I hope you are doing well. 😊❤