Responding first to the title of your message, before reading the question itself:  If you HIV test was done sufficiently long after the last possible exposure, I'm sure you can trust the results. The HIV blood tests are among the most accurate diagnostic tests ever developed, for any medical condition. Done long enough after exposure, negative results overrule all other considerations. No matter what symptoms are present and no matter how typical they are for an HIV infection, and no matter how high the risk of HIV at the time of exposure, a negative result is always proof the tested person does not have HIV. The only exception is when HIV drugs have been taken to prevent infection, i.e. post exposure prophylaxis (PEP).

Now I have read your question. Guess what? You do not have HIV. Or syphilis either. In addition, the sexual exposure was low risk:  a partner who probably did not have HIV, plus the condom. And your symptoms are not typical of either HIV or syphilis; your illness and later symptoms ave nothing to do with the sexual event you have described. And the symptoms started too late:  HIV symptoms always start within 3-4 weeks, usually 10-20 days. To your specific questions:

1. No. There is no possibility your HIV test results are false negative.

2. The second part of your question is mistaken. You have to consider the statistical chance of infection, not the numbers. Compared with numbers of hetersexual exposures -- mostly vaginal sex -- the number of HIV transmissions is tiny. If a female has untreated HIV, the risk to her male partner has been estimated to be under 1 chance in 2,000 -- and that's without condom protection.

You'll be less likely to be misled about HIV risks and transmission if you limit online searching to professionally run sites (like medical centers and provincial or national public health resources) or at least those that are professionally moderated, like this one. Disregard online sites by and for people with the health problem you are concerned about or at risk for it -- like Reddit, for example. Anyone can write anything they want and the amount of misinformation is very high.

Anyway, you can move on with no worries about HIV or syphilis. See a doctor or clinic if you continue to have bothersome symptoms. HIV or syphilis are not the cause and you should not have any more tests for them

I hope this reply is helpful. Let me know if anything isn't clear.

HHH, MD

you are correct: Reddit and other boards are one of the reasons for me being that nervous and anxious about my health. It is good to hear that you do not think that I am HIV positive.

I'd like to follow up with these questions:

1. As personal experiences usually help me: Did you ever hear of a 4th gen HIV test "turning positive" after being negative after 12/13 weeks or did you ever see such a case yourself in your career?

 2. I read a lot of stories about "Late Seroconversion" or even undetectable variants (I guess you already know where I read that). Is that a thing? I mean: In real life, not theoretically like "theoretically I could win in the lottery 4 times in a row"

3. Are "symptoms", you describe them as 100% not HIV-related, possibly caused by anxiety? Is it possible that my mind causes my body to have a dishidrosis or canker sores? What is your professional opinion/experience about that?

2. The "lot of stories" you found are very old or whoever wrote them was simply wrong. Delayed positive test results were an occasional problem for the earliest HIV tests back in the late 1980s and not a problem with any HIV test used in the last 30+ years. With the usually standard AgAb (4th generation) blood tests, "seroconversion" itself if an outdated term. That word applies only to antibody testing, and not to the HIV antigen component inherent in the AgAb tests.

3. Anxiety can have a powerful effect on symptoms. It can greatly magnify minor symptoms or even normal body sensations that otherwise would not be bothersome and perhaps not even noticed. And the very wording of your question suggests it might explain some of your symptoms:  when someone suspects their own symptoms to have an emotional or psychological origin, very often they are right!