1) How likely is it that I contracted oral gonorrhea from the encounter and that changed into disseminated infection? I never had any of those dissemination symptoms (arthritis etc)
Very unlikely. It's unlikely that you'd get oral gonorrhea from cunnilingus, and disseminated gonorrhea (DGI) doesn't change into a genital or rectal infection. It would give you other symptoms, like inflammation in the joints.
2) How likely is it that I got something orally and self innocuated into rectum?
This does not happen. Self-innoculation, or autoinnoculation, means when you infect yourself somehow in another location. For example, if you have a new herpes infection, touch a sore on your genitals, and then touch your eye, you could infect your eye. It's rare, but does happen.
If you had oral gonorrhea, it would be in your throat. You'd have to put your fingers in your throat, then insert them into your rectum. I am sure this didn't happen. And as I already stated, it's not at all likely that you got oral gonorrhea from cunnilingus. It's far more likely from oral sex on a penis, because the act of that puts the penis in your throat. Giving oral sex to a vagina doesn't put anything into the throat.
3) How likely is it that it could be anything else?
It's very likely that it's something else, because the chances of it being gonorrhea are extremely slim. I don't know what it is, or even if it's actually anything. Sometimes, things just itch. If your elbow itched, would you be worried about that?
If it continues, work with your doctor.