Cubby does the same thing when his anal glands need to be drained. I would take him to the vet either way. If could be an abscess like catlover4ever says. Or it could also be his anal glands. Or at least in my experiance it has always been anal glands. Is your cat licking the bottom of his tail? Or is it more towards the middle? Cubby would also scoot his bottom on the floor. After the first time (which I thought was cute) he would cry and do it so I knew something was wrong with him and took him in. Either way take you cat to the vet. And please let us know what they say.