The first clue is the fact that Castiel looks at Sam is though he just asked a very silly question. The fallen angel waits a moment, as if expecting more to come, then shakes his head finally. "I'm fine."

It might just be that he's dehydrated to the point where his bladder didn't quite fill up.

Or, more likely, that whether it's for washing up, for relieving himself or for any other reason, he adheres to commands and shame.