I do not have crohns either, but am recovering from an rectal abscess which was a little different than yours (came on fast, extrememly painful and made me very sick). At the time I was told that antibiotics alone canNOTtreat an abscess. It must be emptied/drained to heal. The wound should also be left open some to continue draining afterward. Also, if left untreated the infection can invade the rest of your body and make you very very sick.. so I vote for you going to the hospital.
I found out from reading this site that you should make sure that the doctor who operates on your abscess is recto-colo surgeon.
hope this helps