The Swansea City captain could not hide his disbelief after seeing the referee point to the spot after blowing his whistle for what he initially assumed was to be a free kick for a dive.
Dorrans converted the penalty to give the Baggies the lead and eventually the three points much to the disgust of Monk.
He told BBC Sport "We knew it was always going to take a dodgy decision to break the deadlock and unfortunately it went against us
"It's gutting really because it changed the course of the game
"It's not becoming a surprise now
"I hate moaning about it but it seems to be game after game now that big decisions are going against us.
"I was right behind Angel at the time and knew he [Barnes] had dived so I was thinking the ref was blowing to give us a free kick or something like that, and when he pointed to the spot, I couldn't believe it.
"We've just watched it back on the laptop and if anything it's a yellow card for the lad for diving, so it's hard to take."
This article first appeared on JACKARMY.net.