Kissing doesn't transmit any STD. Receiving fellatio is indeed low risk for HIV and all STDs -- not zero risk but much safer than unprotected vaginal or anal sex.
Thank you so much for your fast reply. I forgot to mention that we did a bit of kissing too, but that is low risk of infection (no risk of STD, right?). What I find concerned is that the girls who do erotic massage (but no full sex) seem to be ok about you performing unprotected oral sex on them (I have been offered it a few times, but never accept the offer). I guess they have convinced themselves the risk is very low, but in my mind it is much more than low because the number of times it is performed on them from men they do no know the history of.
Thank you again
Welcome to the STD forum. This should be easy and perhaps brief: Before I even look at the question itself, the answer to the title is not to worry about it. They call them STDs because you have to have sex to transmit or catch them, and for disease transmission purposes that usually means a penis inside a vagina, rectum, or mouth. No STDs are transmitted, except with extreme rarity in very special circumstances, by "kissing / fingering / sucking".
Now I have read the question. Guess what? There is no STD risk from the events you describe, regardless of oral exposure to vaginal fluid on fingers (even overt cunnilingus carries little STD risk) and regardless of mouth ulcers (which sound like canker sores). As for your sore throat, you are correct it could be a coincidence. It also could be a garden variety cold virus you caught from the escort. Now THAT is an infection those exposures could easily transmit. No STD is.
I don't recommend any STD testing on account of this event. However, everyone who is sexually active outside a mutually monogamous relationship should have routine STD/HIV testing from time to time, like every 1-2 years. If you haven't been tested recently, perhaps this would be a good time, since it's on your mind. But not because of this particular event.
Really, don't worry about it. You had the safest kind of sex there can be, apart from masturbation.
Best wishes-- HHH, MD