That is so awful, the poor little thing.... I am going to look up some things myself but I came across this and I think it may be Inflammatory Bowel disease or either the kitten has problems with its anal canal. It tells of it but if it is leaking a yellowish fluid, that is infection and the mucus lining is inflamed and raw or either it is the anal canal not working properly. I really do hope this helps.
http://www.vet.cornell.edu/fhc/resou...chure/ibd.html
http://www.sniksnak.com/cathealth/analsac.html
I would not wait to get it looked at, but you said it only hurts when you clean it. In beagles and dogs, their glands must be cleaned and squeezed. They hurt and the dog will yelp and that is the only sign it will show. It will do everything else as it always does, the same goes for a cat who's anal canal does not close well. I am think this is the problem but I am not sure. I hope either of the links help, I will still look about.
I hope everything is well and that it is not serious.