Rovers were already trailing Martin O'Neill's side courtesy of John Carew's opener when Ryan Nelsen hauled back Gabriel Agbonlahor in the penalty area.
Hughes admits Phil Dowd was right to award a spot-kick, but disputed the decision to red card the New Zealand international for the second time this season.
When asked was he aggrieved at the result, Hughes said: "Yeah to say the least.
"We've lost 4-0, but no way was there a four-goal difference between the sides.
"The turning point was the penalty. It was a penalty, but I don't know why he sent him off. We had people back there.
"Once he sent Ryan off that was the end of the game."
"I've calmed down now. I've got to try and curb it."