The monsters patrolling the maze at night, the boys coming out delirious about being stung after the serum has taken action and after the changing, the venom from the monsters, it is all part of the story.
The creatures that were doing the stinging in the maze occurred at day AND night.
Bio-Tech creatures, created by WICKED (World In Catastrophe Killzone Experiment Department) were used, they were made by as their scientific name 'Bio-Tech' suggests, part machine and part organism.
These creatures looked like a bulbous creature:
'It looked bulbous and as big as a cow'
as Thomas (who's real name is Stephen but manipulated to believe hs name was Thomas) describes in the book, and the fact that it was like blubber and fat all piled into one. To support what I am saying, this is the link of what they looked like:
People here are confirming that they only come out at night, but Gally was stung on the otherside of the scale. He broke the Glade rules by stepping out of the Glade at day into the maze just to get the feel of it and out of nowhere, a Griever comes out of nowhere and rolls all over him, he is found soon after.
The griever is known to come out at night but because boys have been stung in broad daylight suggests otherwise because Alby was stung the same way the Gally was, only not in the same spot and the Griever was laying around like a dead stuffed toy and motionless until touched by Alby, there the Griever rolled all over him and he was thus...stung, and because of this, they get shut out in the maze at night.
This is what has been doing the stinging in the maze at night AND day.