I really don't have any expertise regarding the CT finding the fistula. I have a Daughter your age, so I just wanted to reply to offer some encouragement. I'm not too familiar with Biologics either, but maybe you could stay on the Humira a while longer to see if things improve.
I'm pretty sure an MRI will give you a very accurate idea of whats going on.
What do your Parents think about what you should do next?