Yeah, I figured that was the problem If the hygrometer was at the right place and still reading 40 it'd have to be really dry. Get it to the right place just to make sure, but I'd bet your humidity is in the right range.
They can retain water in a dangerous way, but that looks different. Like, you know how the look puffed up right before they shed? If they look like that when they aren't about to shed or puffing up because they're mad/scared then that's a problem. It's normal for them to hold enough water that they visibly shrink when they pee.