I have crohn's colitis as well (CD affecting the colon where ulcerative colitis typically hangs out) and I have severe perianal crohn's skin tags...mine are definitely not hemmies as they were mistaken for being hemmies and I had them banded while they were in a bad flare (large and hard), then all hell broke lose with my CD internally (oddly enough my CD started with the perianal crohn's skin tags before it started affecting my intestines).
With having crohn's colitis we can be more prone to bleeding, and although you've had some constipation issues (I too have been there) bleeding can come from inside and not necessarily the "bump(s)" you're describing (my cron's skin tags have never bled, but I've bled from inside).
It's common for the skin tags to become swollen (large, hard and painful) especially during a flare or the start of a flare.
An abcess often has a pimple look to it (with a white head) but I can't say 100% cuz luckily I've never had an abcess or even a fistula (knocking on wood I never do).
Here is a link showing pics of hemmies and below it, pics of perianal crohn's skin tags...you can see there is enough of a difference between the 2 (which is why I'd like to shoot the idiot surgeon that banded mine and for mistaking them as hemmies). {{{{THE LINK IS NOT WORKING, so I cannot post if for you, I will try again later for you}}}}}
It's really important with having crohn's colitis that you take a fibre supplement daily and indefinitely as it aids with either D or C, my GI suggested it to me yrs ago and it was the best advice ever...check out my signature below.