About 4 years ago I had a sexual encounter in Spain with a sex worker (south american). I had unprotected receptive oral sex and protected vaginal sex two times (protection first time 100% sure, second time I would say almost sure, but anxiety, drunk memory and the time passed are playing with my mind. 

Regarding possible symptoms, the morning after, my mates and I got some sort of stomach virus (vomiting,diarrhea and fatigue) that seemed to get better fast but gave some trouble for about a week (probably irrelevant). Also, I would say exactly about a month after, my pitiriasis versicolor in my neck and back área extended itself significantly. Its important to highlight that I had been developing it the month prior to the encounter, it just got worse on that particular time (got it treated afterwards). This might also be due to the amount of physical activity and sweating that I had daily. Being a bit ignorant on stds I never felt I had been in danger but anxiety over It has grown tremendously over time (affecting me physically and psychologically, which lead to overthinking) and got tested today (results next week). My question is, should I have any concern on the result being positive given the exposure and time passed since then? Thank you

First, you describe a quite low risk exposure, since STIs in general are uncommon from oral sex, and condoms are highly effective. Second, the symptoms you describe were not possibly due to the sexual exposure; no STI can cause symptoms in under 24 hours, and gastrointestinal symptoms don't fit with any STI. Third, absence of urethral (i.e. penile) discharge in the next few days is nearly 100% assurance you didn't acquire gonorrhea and reasonably good evidence against chlamydia. But at this point, these don't matter:  the immune system would have cleared either of them long before four years, so there was no need for testing for them at this time. 

Pytiriasis rosea (not the same as P versicolor, but similar) can be very similar to the rash of secondary syphilis, but 1 month is too soon for the secondary stage, which typically shows up 2-3 months after acquiring infection. That said, it would be reasonable if your current testing includes a syphilis blood test. But you definitely can expect negative results.

As for HIV testing, I would judge that there was no need except for reassurance -- this was a no risk exposure in regard to HIV, which has never been known to be transmitted oral to penis; condoms are highly effective against HIV transmission by vaginal sex; even without a condom, if a woman is infected, the risk from a single episode of vaginal sex is around one chance in 2,500; and statistically, the chance such a partner had HIV is very low. So you definitely can expect a negative HIV test result.

I hope these comments are helpful. Let me know if anything isn't clear.

HHH, MD