A bit more info could help, but plants droop like this when they sleep and it can be exaggerated if you water close to lights off.
It can also happen if you let the soil dry back quite far and then give a big dose of cold/cool water in the middle of their light cycle.
It's like you sucking hard to get the last bit of milkshake out the glass and then someone throws water in your glass and you get a massive mouthful of water and your cheeks puff up from the sudden pressure change.