I am on prep and have been taking my meds consistently everyday. But recently I had unprotected sex with a guy who was HIV positive undetectable. Even though he told me it's undetectable but since I don't know him, I can't be sure. I have been very stressed out.
I have some good news for you: There is absolutely none, zero, zilch, nada, ninguno risk of HIV in the encounter you are describing!
First thing we want to change here is how you are using the term, "unprotected sex". If you've been taking PrEP consistently every day then you are taking responsibility for having the most protected sex one can possibly have. PrEP is far more effective than condoms in preventing HIV, So educators, as well as governmental agencies like the CDC, consider daily use of PrEP to be "protected" sex. (https://www.hivplusmag.com/research/2014/02/13/cdc-stop-using-unprotected-sex-condomless-sex).
All of this is relevant and true regardless of your partner's HIV status. But, in this scenario, you are describing sex with a partner who is HIV positive and undetectable. If they have been undetectable consistently for six months or longer, there is zero chance they can sexually transmit HIV to others (https://www.preventionaccess.org).
So your stress here doesn't seem to be derived from medicine or science; it has more to do with the life-long (or 37+ years) messaging that condoms offer the only form of protection from HIV. Those of us who have been working in the field for sometime (for me since 1991) spent decades drilling into your mind: Condoms offer the only true protection from HIV.
Now we know that isn't true. Now we know PrEP alone, and HIV treatment alone, bring risk down to zero. But when you combine PrEP together with HIV treatment, there is simply no risk. If there was such a thing as less-than-zero risk I'd propose it here, but medically we can only say "zero" risk.
To learn more about PrEP, risk, and overcoming 30+ years of fears, please check out my Facebook "PrEP Facts" group at: https://www.facebook.com/groups/PrEPFacts/
I hope this information allows you to relax, and understand that if you're having sex, PrEP and/or HIV treatment are literally the safest means possible. Enjoy!