Disease in my anthias

Hey guys.......has anyone ever had a fish that developed a swollen anus.  I just noticed today that my anthias has this problem.  It looks to be mostly white in color but in that area it is generaly white anyway, and the scales seem to be kindof jutting out because of the swelling.  In the center there seems to be something white poking out.  It almost appears like it may have eaten something the size of a pea and just couldn't digest it and it is trying to pass it but it is having a hard time doing so.  The fish seems to be acting normal.  Does anyone have any idea what the problem could be?  Bacteria, viral.......what?  If anyone has seen this before or knows what it might be let me know.  Thanks

Well the fish died the next day after discovering the problem.  The strange thing about it was that it seemed bloated, as if it had a full stomach.  After it died I cut the area from just in front of the anus down to the anus and popped what seemed to be its air bladder.  I always thought that fishes air bladders were closer to its spine.  But maybe it had some gas build up in its stomach, I'm not sure, or maybe that is where the air bladder is on this fish.  I was looking to see if maybe there were some type of parasite living in it that might have been causing the problem but I'm still unsure what it could have been that caused the problem.  It never ate anything that I could tell in my aquarium but what you described may have been the problem.  However, it did die rather quickly after discovering that there was a problem.