Hi colitiscanuck...
UC can also cause
weight loss (constant D can do that and generally with either IBD it can cause weight loss so that alone isn't an indicator for which IBD one might have).
It's rare, but some UCers also develop fistulas but it's usually seperate from them having UC where as with CD it's usually related to it, and more common.
The main differences are the type of inflammation one has...patchy inflammation is indicative of CD, if the entire area is inflammed with no patches, that's UC...also with CD the inflammation can go through the many layers of the intestinal lining, with UC it's limited to the surface of the lining only. Of course
location to determine which type of IBD counts too (like you I also have crohn's colitis) UC is limited to the colon/rectum, CD can affect any part of the GI tract from mouth to anus and more than one area at a time as well.
It's generally constant prolonged D that causes the weightloss over other IBD symptoms (which is why weight loss can also happen for UCers as well).
You can try drinking meal
supplements on top of eating to help gain weight...I hope your GI can aid you with your fistula, I've heard Remicade can help heal them quick.