You may be suffering from one of the following possibilities in the descending order of probability:
1. An abscess
temporarily complicating an anal fistula
2. An abscess temporarily complicating a pilonidal sinus
- ONLY IF the spot that you have described is behind the anus (and not in front).
3. Infected sebaceous cyst
4. A simple incidental boil in the skin.
Whatever it may be, at this stage you need to take appropriate antibiotics and analgesic
(as prescribed by your doctor).
The exact diagnosis will be made mostly at a later stage - i.e. after the acute infection has been controlled and re-examination is done.