I just realized why this song seems familiar – it is strikingly similar to Homer and Jethro's song I DON'T THINK MY GAL LOVES ME ANYMORE which I posted 9 months ago. That song was recorded in 1958, 9 years before THAT'S WHEN I GUESSED (YOU DIDN'T LOVE ME ANY MORE). The tune is somewhat different, but the metrical structure and rhyme scheme are very nearly the same, the title/refrain is very similar, and it's repeated just at the same point in each verse where the title/refrain is repeated in the other song. They are both comical songs in which the narrator reports being mistreated by a woman. Do we need any more proof they are related somehow?

Most, if not all, of H&J's songs are parodies, but in this case, if it's a parody, I don't know the original. Is the date wrong? Are they both parodies of some other song?