Bruce Dyer grabbed the winner as Barnsley came from behind to notch up an impressive victory that pulled them out of the drop zone.
But Chesterfield boss Dave Rushbury admitted his side were being dragged back into a possible relegation battle after their recent slump.
"At the start of the season we got the points early but now we are left looking over our shoulders because we are being drawn back into the mixer," he admitted.
"At the moment we are stuttering but hopefully we will get back to where we were at the start of the season.
"We went in front but then conceded two goals and that's the way it's going for us at the moment. Teams are not creating goals against us we are just giving them away." However, Barnsley boss Glyn Hodges was delighted with the victory.
"I thought we deserved the win," he said. "We were the better side and came from behind to win.
"When they scored first I thought 'here we go again'. We had played some good stuff and been dominating but fell behind.
"But the team responded well and we showed fight to level the score and then get the winner." Chesterfield went in front after 18 minutes with an impressive strike from Chris Brandon. Glynn Hurst curled in a cross from the right and Caleb Folan headed the ball down for Chris Brandon to fire home from six yards.
But Barnsley were level two minutes later when keeper Ben Williams was beaten to a Gary Jones cross by Rory Fallon who headed in from close range.
And in the 32nd minute the Yorkshire side went in front against their Derbyshire neighbours.
Kevin Donovan's cross from the right was headed down by Anthony Kay and striker Dyer rammed home his shot from 15 yards out to claim his 11th goal of the season.
There was more misery for Chesterfield when, four minutes from the end, Steve Blatherwick was sent off for a second bookable offence after he brought down Fallon when the Barnsley striker was clean through on goal.