In Valtheim Towers there's a potion of minor healing next to a bucket lol.
Heh, its to help with the pain just in case if you ate something spicy the other night

Anyway, havn't you seen those people grasping their butts, jumping up and down, running into the woods? Follow them and they'll lead you to it... Or they just got butt [censored].. who knows?'
Seriously though it is the forest or a bucket.
Speaking of the Dovahkiin, he just does takes a dump while your in 1st person.