It looks more like the water bottle dripped on some poop and maybe a pellet? If that is the only place you see it that is likely what it is. Another possibility if it's just the one poop, is it could be cecotrope, which is a special kind of poop chins eat to basically recirculate the good gut bacteria and any nutrients they didn't digest the first time through, it's also softer then their normal poop. Normally chins eat it pretty much straight from their butt so you never see them, but sometimes they miss one. If a chin has diarrhea you would notice it on multiple surfaces since they poop about once a minute.
Just so you know, the issue with cardboard is if they swallow it it can cause a blockage, so you would see little to no poop, or small poops, and you will normally see an issue if they are going to have one with in about 12-24 hours after eating it.