I had 2 yellows after the final whistle today, one was a second yellow so became a red. I'm literally at the start of my first of 3 blasts to blow for full time when a player goes straight through his opponent, clear yellow card, the fouled player (who was also on a yellow) jumped up and then went over to the offender and squared up to him, face right up close and screaming at him.
Yellow for the original offence and yellow for adopting an aggressive attitude for the responding player, which just so happened to be a second, so quickly followed by a red!