Hi Filipa,
I know it's scary when new things pop up, but you will be ok.  The MRI will give good information.  Your doctor may recommend biologics (remicade, Humira, Cimzia, etc) if they suspect your immune system is causing the fistula formation.  People have had good responses with these.
Hope all goes well for you!

Thank's for the answer, for now they believe it could be from the dilatation I had to have, they that it could have ripped something inside... I think Monday I'll have some news after the MRI.

before I had my colon removed I tried biologics with no success, remicade, humors, done them all, so I' m hopping they won't be necessary... Let's see...

They might recommend a seton. I have one right now and it has really helped with the pain and inflammation. The recent studies seem to indicate that a seton followed by one of the anti-TNF alpha biologics has the best chance of getting rid of a fistula. Unfortunately they are horribly difficult to get rid of and none of the treatments have very high success rates. Good luck!