Or if it's in the "crack" area it could be an abcess or soon to be fistula....external hemmies stick out of your anus, internal ones don't.
Check out this link, the first set of pic shows both kinds of hemmies (internal and external) the pic below that shows perianal crohn's skin tags (only related to crohn's not UC) doesn't hurt to look, afterall tons of people seem to get misDX between UC and CD....you can also google pics of "anal abcesses" and anal fistulas to see if any of those pics resemble what you have.
Here's the link I was referring to...
http://www.emedmag.com/html/pre/fea/features/011504.asp
:)