Hi
Let me know if I got this wrong but is sounds like the condom slipped off while you were having vaginal sex.
The concern would be if you came in contact with any vaginal fluids when this happed, I could see that depending were the condom was when it slipped off that your penis may or may not have come in contact with vaginal fluids.
It sounds like you have been really safe with using condoms for vaginal/oral sex and going for STI testing on a regular basis. As it is possible that your penis may have come in contact with vaginal fluid I would recommend going in for a STI test, as this would be like having unprotected vaginal sex in terms of risk. In terms of the chance of something passing this is hard to know as it really depends if the person has an STI in the first place, even if they have an STI it is never 100% that it will pass the one time you have sex.
STI like chlamydia and gonorrhea usually show up in a urine test by 2 weeks so it best to wait at least 2 weeks before getting tested unless you develop symptoms e.g. discharge from penis, hurts when you pee, sores on the penis etc.… Blood tests that check for STI like HIV, syphilis and hepatitis B usually show up by 6 weeks on a test but can take up to 3 months to get a final result.
Given this I would recommend waiting 2 weeks before getting a STI check and then go back at 6 weeks to repeat the blood test. I would also consider getting the hepatitis B vaccine if you have not had it already. We know it can be a stressful for some people when they have to wait to do a test and then wait for the results. If this is happening for you, let us know if you need any help or support during this time.
In terms of using condoms in the future having a condom that fits better (feels comfortable and you can roll it down all the way) can decrease the chance of it slipping off and we also find that some people like using a female condom if they are having a problem with a male condom fitting.
Let us know if this does not answer your question or if you have any more questions or concerns.
Health Nurse