Sorry you aren't feeling well. It is cold and flu season. The reason I say that is because think about this . . . Hand jobs are no risk. You do not get a hand job from giving or receiving a hand job. Hands do not transmit std's. No risk. He gave you mere seconds of oral. This is unlikely long enough for any type of std. AND, your scenario of his going own on you and hours later YOU have a sore throat . . . that has nothing to do with getting an std from receiving oral. Symptom off and timing off. That's just a sore throat likely from a virus. That you got BEFORE this encounter (viruses have incubation periods). Kissing is a risk for common germs that give us cold and flu. Hygiene overall isn't related to someone having an std. You will never be able to look at someone's appearance and judge if they have an std. Because (you didn't) if you had gotten an std, does your appearance have anything to do with it? Can you wash it away? No. So, that fact is irrelevant. But glad there were no sores.
I think you are likely over worrying. Your risk for receiving oral sex for seconds was scant at best and you'd only be worried about what was in his mouth in the off chance he had some kind of mouth std going on. kissing is a risk for ora herpes but 3/4 of the planet actually has that (even kids). So, I would not be worrying about this.