I had a peri-anal fistula... It started off as a slight swelling under the skin, then became more swollen, like a hard bean under the skin, then it started to soften up before it burst open...
It does sound like you might be experiencing the same... It was VERY painful, sitting down, getting up, changing positions was excruciating. Have your doctor take a look at it, he will probably prescribe ciproxin and flagyl to start with.
Good luck.