A MISERABLE bank holiday weekend saw the Reds suffer back-to-back defeats for the first time in two months.
Having recorded five wins and three draws in his first eight games in charge, Lee Johnson's side went into Saturday's South Yorkshire derby against Sheffield United in confident mood.
Over 17,000 fans flocked to a sun-drenched Oakwell to watch the match, but it was Steven Davies who put the visitors ahead just before half time with a fine header.
The game was put to bed midway through the second half when Jason Holt's 20-yard effort doubled United's lead to consolidate their play-off place.
A trip to relegation-threatened Colchester was next up on Easter Monday and the Reds looked good going into the final 30 minutes, leading through George Waring's 40th-minute goal.
But the game was then turned on its head, with Gavin Massey, Jacob Murphy and George Moncur all finding the back of the net to leave Barnsley seven points adrift off the League One's top six.
Johnson said: "It is disappointing, of course, as we have been on a good run and everybody has whipped up into a frenzy.
"It was almost like a balloon had just been popped. Players started looking they'd had two games in two days, mentally rather than physically. We've given ourselves a lot to do now."
Next up for Barnsley is a crucial match at home to sixth-placed Chesterfield on Saturday.