Newcastle manager Steve McClaren said after the game that the penalty award was "absolutely ridiculous" and Dutch defender Janmaat backed his boss to the hilt.
"I think we were by far the better team in the first half, and then we get a red card like this. It was unbelievable," the 26-year-old said, quoted by the Chronicle.
"I don't like to blame referees, but that was not a penalty. I think everybody saw it - and a red card is crazy in my opinion. It kills the game.
"I don't like excuses, but I think we had 70 per cent possession in the first half and were far better than them.
"In the second half, we lost the game with 10 men. It is disappointing because in the first half we played well."
The defeat left Newcastle second from bottom with six points from their first 10 games, a place below Sunderland, who have a superior goal difference.