Neither herpes nor any other STD can cause pain, soreness, or urinary "difficulty" immediately after sex; those symptoms had to be some sort of trauma or irritation from the sexual exposure itself. Also, herpes does not generally cause redness without visible lesions, nor does it cause itching or tingling of the scrotum or anywhere else. You cannot catch gonorrhea or chlamydia through a condom, nor chlamydia from oral sex -- testing for those things might have made sense just to be safe, but if they had been positive, it would have come from some exposure before the escort event you are concerned about. Finally, as you suggest yourself, diarrhea certainly can be an anxiety symptom -- and when a person suspects his or her own symptoms are due to anxiety, usually they are correct.
I never guarantee someone isn't infected. But from all you describe, I very much doubt you have herpes or any other STD from the escort exposure. Continue to follow-up with your provider if your symptoms persist. But if they fade -- as they probably will -- I would just let it go and move on.
Good luck-- HHH, MD
Syphilis is rare, with only about 20,000 cases per year in the entire US, three quarters of them in gay men. And syphilis doesn't cause any of the symptoms you describe.
I have no other ideas or advice. If your symptoms persist or you remain concerned, continue to follow-up with the provider who did the STD tests, or with your primary care provider (if they aren't the same person).
Since you ask about prostatitis, I will say that I doubt it. However, you might google "chronic pelvic pain syndrome", which used to be considered a form of prostatitis. Start your reading with the excellen Wikipedia article. As you will see, CPPS would also fit with my guess about a psychological cause of your symptoms.
Don't ask your provider for any particular exams or tests. Just explain your symptoms and follow his or her advice after that.