The Lilywhites midfielder was booked early on for a foul on Jacques Maghoma before being warned by referee Scott Duncan in the first half.
Pearson was penalised for more fouls in the second half with Monk staggered that the 24-year-old wasn’t dismissed before Sean Maguire popped up with a 94th-minute winner.
“How they’ve not ended up with 10 men on the pitch is beyond me,” Monk said.
“I don’t want players to be sent off, I’m not here for that.
“But how a player gets warned in the first half for making so many tackles when on a yellow card and he doesn’t receive a second one in the second half is just, wow.
“I guess that sums it up.”
Monk felt his side did more than enough to win a game that was low on quality throughout.
Declan Rudd was called into action with a string of key saves, Maghoma also missing an excellent opening in the first half.
“We were the best team on the pitch,” the Blues boss said.
“We totally controlled them and I can’t remember them having a meaningful shot on our goal until the end.
“The frustrating part of that performance was we didn’t take the chances we had.
“We should have been walking off that pitch with three points but it’s the classic sucker-punch.
“We dealt with the counter attack but then from the corner it’s about individual battles and unfortunately we lost one and it’s cost us dearly.
“To walk off the pitch with nothing is a very harsh result to take if I’m honest with you.”