It is hard to tell if it is an abscess or a fistula. The thing with an abscess is that you don't want a deep one breaking inside of you. Since you can see this bump, and if it does break open, it is going to drain out, so you won't have the infection going inside of you.....that is a good thing. I would definitely get it looked at, but not at the er if you can help it. They will just want to slice it open and drain it themselves....I am speaking from experience here. Don't let them do it....it will just come back. Like Glad Bag said, take your pain meds if you have some, if not, call tomorrow and get a script for one, or if it gets too bad, go to the er for meds. You can't really go by the low grade fever thing....I have never gotten fevers with mine. It would be so much easier for all of us, if each problem would present itself as a classic txtbook case, but as you know that doesn't happen! If it is an abscess, make sure you have it surgically drained. Good luck! Keep us posted.