Well, that sounds uncomfortable.
For starters, I'm not sure you want to use wound. You get a wound from being wounded, which is to say, accidentally or deliberately pierced or cut by some object. While it's certainly possible to be wounded on your anus, that would probably make an interesting story.
Instead if you are talking about some other kind of medical condition, lesion is probably the more accurate term:
lesion (n): A region in an organ or tissue which has suffered damage through injury or disease, such as a wound, ulcer, abscess, or tumor.
Lesion is kind of a medical term, so more commonly, and depending what it looked like and what caused it, you would describe it as a blister, boil, cut, bruise, sore, rash, tear, or just an infection.
If you are talking to a doctor, I would recommend being specific about where it is. Otherwise, if you are speaking euphemistically in (relatively) polite company, then "on my bottom" is close enough. Few people really want to know too much detail about something like that.