It certainly could be an abscess and fistula. I am glad you have a doctor who is taking it seriously and you will have a scan. The draining and filling sounds like what I had. Antibiotics helped but never got rid of it until it was opened up and drained (and in my case, two setons installed). It is amazing how fatigued you get with abscesses. Our bodies must spend tremendous amounts of energy fighting the infection. I suggest lots of hot sitz baths to help get the infected stuff out of there.
Let me go on the record again -- I HATE ABSCESSES!! I hope you get rid of that nasty rude thing fast!
Diagnosed with Crohn's in early 2007. Several peri-rectal abscesses and two fistulae with setons. Allergic to Remicade and Humira. Currently on 6MP, and vitamins D and B-12.