A couple of years ago, I had--in this order--a perianal abscess, a perianal fistula, and a nasty fissure. I was totally panicked, since I thought this meant that my disease (which had always been confined to my terminal ileum) was spreading into my colon and rectum.
The colorectal surgeon told me that that wasn't the case at all. Turns out that there are plenty of mechanical reasons for all three of these things, including periods of very bad, irritating diarrhea that can cause an abscess, fistula, or fissure. I was really surprised when he explained to me that lots of people without IBD get all three of those things. (In fact, my husband, who doesn't have any chronic disease or conditions, had to have surgery on a very painful fissure last month.)
The surgeon lanced and treated the abscess, which disappeared. He was able to surgically repair the fistula. But nomatter what he tried, he was unable to get the fissure to heal--so I have it permanently now. Sometimes it gets irritated and really hurts (if there's been lots of watery diarrhea and straining); the rest of the time, it's mildly bothersome. But I can live with it, I guess because I have to.
Five years later, my CD is still confined to my terminal ileum. So a fissure doesn't necessarily mean that your disease is spreading.