Dean, Sam knows, is very good at pissing people off. He should know, considering that he's Dean's target on a fairly regular basis, and that's when Dean's being intentional.
Therefore, he shouldn't be surprised when his brother's "girlification" (as Dean put it) has been caused by something along the lines of this:
Dean hits on a girl at the bar (as he is almost guaranteed to do; it's nothing personal). Said girl takes offense and sneers before sauntering off, muttering something that sounded suspiciously like "fucking douche" underneath her breath. Sam suspects she might've been a witch, or a witch might've been in the vicinity at the time. It's kind of a toss-up at this point.
The point is, Dean's current predicament is pretty much his own fault.
And yet if that's the case, why is Sam the one stuck listening to Dean complaining about the indignities of peeing sitting down?
Life, he thinks with a little sigh, can be so unfair.