This is the feeling of sadness after you just realize you got sloppy seconds. You thought you were getting her wet but point in fact you were stirring some other guys jizz.
"I just got the sloppy second blues after I realized that Bob fucked Ann minutes before I put my c*** in. I had to burn my clothes and scrub down with gasoline."
When multiple couples are friends on Facebook and male A shares a meme with female A, then male B, seeing said meme, shares it with female B. Female B realizes that said meme was not exclusive to couple B.
Mary thought John sent her a sweetmeme, then she saw that Mike sent it to Jill an hour earlier. It was then she knew she had fallen victim to sloppy second sharing.