[Based on "transposition" by wildwestdean on Tumblr!!!]
It's been sort of rough, figuring this whole witch thing out on you own.
You've been fighting a learning curve while on the road with two hunters and with no steady mentor, not to mention the whole 'Dean hates witches' bump in the road that lead to a huge argument that you're only now recovering from.
So you decided to do something nice for him and cast a spell to help out with one of his hunts, but something must have went wrong, because suddenly the floor looks a lot farther down and you're looking at your own face.
It hits you after a few seconds. You and Sam swapped bodies, which was absolutely not your intention with this spell.
And Dean's knocking on the door now, and you're both so screwed.