Interesting....I had something similar a couple years back. The doctor gave me some yeast infection pills and cream and it went away. I didn't even think about it being a yeast infection, because it was on my bottom. The doctor said that sometimes fungus from a yeast infection can spread to the outer skin around the vagina and to the back...especially because we are supposed to wipe from front-to-back.....so if there is an infection such as a yeast infection....it may spread.

Hope this helps.