The abscess and the fistula are likely part of the same process. Fistulae often go through cycles of draining and getting clogged up. When they are clogged they will swell, hurt, and pretty much act like an abscess. Then the “abscess” drains, and may keep draining a bit until the opening clogs up again. A draining seton keeps it open all the time.
This may not be your situation, but it’s worth Knowing about.