The result puts Rovers within striking distance of their hosts at the bottom and undoes the hard work that the Reds had put in over the past three games to haul themselves out of the relegation zone.
"It was a disappointing performance and we didn't show any signs of that in the last three or four games - none whatsoever," said Cotterill.
"We had a couple of injuries before the game with Marcus Tudgay not able to start and Paul Anderson unable to take his place on the bench. That might have upset the dynamics but right at this moment I can't explain what went wrong.
"I know one thing - I am pretty angry at the moment but I will lift myself up. I'll be fine.
"The players would have been in tomorrow anyway but we'll work with them and try and recover as quickly as we can."