I have read responses to similar questions but wanted to ask mine regardless. Me, 49 yo male. I have just met a girl and we decided to both get tested for STD:s which I did last week, everything came back negative. Well, there was this other girl who I met through a dating app few months ago. She is 36 yo, single for a year after a long relationship. I received oral sex from her a few months ago, that would have been covered by the test last week. Now there was a bad choice of meeting her again yesterday and things led to her again giving me oral sex for a few minutes. I know this is low risk, but today I got the worst stomach flu, have been sitting in the toilet with diarrhea and throwing up a few times. Not a hangover, I hadn't been drinking. No other forms of sex, some kissing and fingering. 

I probably wouldn't have thought much about it without the diarrhea and feeling sick. She said she had been tested 6 months ago, but I can only assume she has met other people after that. We live in Western Europe for what that's worth.

Since I just tested negative last week, I wouldn't want to wait for 6 weeks for the HIV test to be conclusive. I know that receptive oral sex carries low risk, would the diarrhea be something to be worried about. It happened the day after oral sex. Also, if I get no symptoms within the following few days, should I be confident that no other STD:s were transmitted? Basically, how relaxed should I be and would you recommend testing at all and could I continue unprotected sex with the girl I met without putting her at risk if I get no symptoms within the next 5 days?

The casual encounter you describe did not put you at risk for HIV- there are still no proven cases of HIV acquired from receipt of oral sex.  As for other STIs, the risk of having acquired anything is very low.  Most people do not have STIs and most single encounters with infected persons do not result in transmission of infection.  This is particularly true for receipt of oral sex as most persons do not have oral STIs and when they do, transmission of infection occurs rather rarely.   When STIs are acquired from oral sex, far and away gonorrhea is the infection most commonly acquired and over 90% of recently acquired gonorrhea becomes obviously symptomatic in over 90% of infected persons, typically within 3-5 days of exposure.  

Diarrhea and vomiting would not be expected to result from receipt of oral sex.  

Personally, I would have little concern over the casual encounter you describe but that, of course, is a personal choice.  Should you choose to test, the important test to get is a urine test for gonorrhea (chlamydia testing is typically done automatically with gonorrhea tests).  Should you choose to test (I would probably not), I anticipate that the test will show that you are not infected. 

I hope that this information is helpful.  If anything is unclear, please use your up to two follow ups for clarification.  EWH

As for many others posting questions here, I guess the anxiety, regret and overthinking takes over, "what if the diarrhea was my body reacting to the HI virus etc."

Then my takeaway is as following. HIV is simply not transmitted in fellatio from mouth to penis (even in the low likelihood that the sexual partner would be infected) and the diarrhea and vomiting a day after the encounter must be something unrelated, a norovirus or whatever. There is a low risk of mainly gonorrhea transmission had my sexual partner had it in the throat. But that would likely cause symptoms in me within 3-5 days.

I will not be tested due to this encounter, unless I develop symptoms next week and will proceed unprotected sex with my hopefully longtime partner to be and try to make better life decisions in the future.

I understand that there is no way to guarantee that somebody is not infected without testing but would the above make sense out of a risk perspective?