The Reds edged further away from the relegation zone with a 1-1 draw at Oakwell which made it three games unbeaten.
It could have been even better had Garath McCleary's first-half goal been added to and Cotterill felt that a 'cast-iron' penalty was not given on the stroke of half-time for a handball by Barnsley goalscorer Craig Davies.
"Goals change games and they change mindsets," said the Reds manager.
"If we go in at two-nil I think we know we're going to win the game then.
"I spoke to the referee at half-time and he said he didn't see it. I don't know what he was looking at then if he didn't see that.
"If that had been us losing and we gone in the bottom three, I'd probably be sat here raging."
Home boss Keith Hill admitted his side were 'fortunate' to be still in the game at half-time and, despite an improvement after the break, to have won the game would have been 'harsh'.