Aww I'm sorry you've been so worried about this for so long because your risk is zero for almost everything.
First, you could have BV. That can happen to any woman, of any age, with any sexual activity - a lot or absolutely none. It does seem to happen more in women with more sexual activity, but virgins can get it, too. It's just simply an imbalance of a certain bacteria in the vagina, and the balance of the vagina is very delicate.
Since you self-treated for yeast, and it felt better then came back, it could also be a really persistent yeast infection. It's a shame they didn't run any testing on you for yeast or BV. With a persistent itch, that should be the first thing they should do.
So for STDs - the only thing that might remotely be a risk for you is genital herpes type 1, and really, if he only performed oral on you for a few seconds, it's not a risk. It takes some pretty serious friction and time to transmit that. That happens when someone who has oral hsv1 (think cold sores) performs oral sex on someone and it transmits to the genitals. It becomes genital hsv1. The number indicates the strain of the virus, not the location.
If he's a virgin, and you were his first everything, he can't have anything except hsv1 orally. A lot of people get hsv1 as children, from well-meaning adults who kiss them, or at day cares or schools when they share toys at the "let's put everything into our mouths" stage.
If he didn't have any visible sores, though, chances of you getting it are very low, whether through kissing or oral sex. I wouldn't even worry about it. If you have hsv1 already, you can't get it again.
The rest is not a concern at all. He can't have it, and oral sex is much lower risk than vaginal or anal sex, and your oral sex was so, so brief. Your doctor was correct that you didn't need to test. For future reference, testing at 5 days is conclusive for gonorrhea and chlamydia. Depending on the test, HIV is usually conclusive at 28 days if you are using the most current tests, and syphilis at 6 weeks. Herpes blood tests are conclusive at 12 weeks.
STDs are transmitted by oral, vaginal and anal sex. Condoms offer full protection for gonorrhea, chlamydia, trichomoniasis, HIV and hepatitis B. Condoms offer significant but not full protection for herpes, syphilis, and HPV. Vaccines are available for hep B and HPV. If you haven't received them yet, talk to your doctor about them.
I don't know what's happening to your mouth. It's not related to any activity with your boyfriend. Maybe it's an allergy to something you are eating or drinking? Your doctor can help figure that out.
So long story short - it could easily be BV (which can cause PID) or yeast. Ask your gyno to test you for those. Don't self-treat again. You had no risk for anything, and don't need any STD testing. Ask your doctor about hep B and HPV vaccines.