What about your exposure was "high risk"?
Based on your timeline... you're saying that last December you had an unprotected oral exposure and the very next day you had penile pain. No STD would give you symptoms 24 hours later. It's more than likely that your pain that quickly was caused by normal heightened sensitivity post oral sex, possibly from tiny abrasions or broken blood vessels from fellatio, or your own anxiety from the experience which can cause physical symptoms.
You say all std's negative... what tests did you do and when?
Were you tested for NGU?
This is a long time from exposure based on your timeline to still have symptoms. I suggest you see a doctor... if necessary just present the symptoms and omit your backstory, it's possible this is unrelated to the exposure.
What were you symptoms of prostatitis?
If you tested negative for the STDs, it wasn't what caused the prostatitis.
The only thing you haven't tested for is mycoplasma. That's a bacteria that can be sexually transmitted, though transmission from oral sex is very rare - https://academic.oup.com/jid/article/216/suppl_2/S396/4040968
There is a possibility that you had the prostatitis before this encounter, and something about it triggered worse symptoms, making you aware of it. There is something called chronic prostatitis, or chronic pelvic pain syndrome, where the cause of it is very hard to determine. Google CPPS in men.