Swollen back end on bantam

[chknaddict]

This little "guy" is a bantam and we thought male, about 9 weeks old now. About three weeks ago, we attempted to introduce him to our other chickens outside and they picked his vent area quite badly. I cleaned it up well and put him back in with some younger chicks and he has seemed to recover just fine. However, he's had trouble with a dirty rear ever since, and today we noticed that his back end is quite swollen, not just around the vent. Any ideas as to what could be wrong and how I can help him? If we're wrong about him being male, he/she would still be too young for this to be an egg, right?

Thanks for any help.