Forest looked to be cruising when two first-half goals from Kris Boyd, one of them a penalty, put them in command by the break.
But the hosts roared back to level through headers from Marvin Elliott and Jon Stead before Luke Chambers nodded home from a corner to give Forest the points.
"We gave away two soft goals and gave ourselves a mountain to climb, despite being the better side in the first half," he said.
"I told the players at half-time that if they got an early goal the game was still there for us to win and that's how it panned out.
"We had some great chances and the winning goal has taken a deflection off one of our lads and looped into the only part of the net we couldn't defend.
"It was a cruel way to lose, but you can't afford to give good sides a two-goal start."