Your partner apparently is confident she doesn't have HIV, and most people don't lie in this situation. On the other hand, if she has multiple commercial or casual partners, and obviously does not insist on condom use for anal sex, she might have been infected since her last negative test. Another aspect -- which you may not know -- is whether her clients or other partners mostly include straight guys (men like you) or men who regularly have sex with men. The latter obviously would be higher risk.

All things considered, I believe you should seek PEP. Assuming that is done, you could start PEP, contact your partner if able, and ask her to be tested now (and perhaps offer to pay for it). If she tests negative, you could stop the PEP after a few doses.

This doesn't mean you're at very high risk. Unprotected anal sex carries an average transmission risk of about 1 in 900 for the penile partner if the anal partner has untreated HIV. Your risk probably would be lower than that, given the brief exposure. And of course the risk is dramatically lower if you can get strong assurance she isn't infected. Either way, the odds are strongly in your favor even without PEP. However, most people would consider a roughly one in a thousand risk much too high and would recommend PEP. All in all, I think this is a situation that is properly concerning, but not one of panic.

Also don't forget other STIs. You should have a rectal gonorrhea/chlamydia test in a few days (unless the ER also prescribes antibiotic treatment accordingly) and a syphilis blood test in a few weeks.

Assuming you seek PEP, the sooner the better. Although 72 hours is the outer window for PEP recommendations, every hour of delay after exposure raises the infection risk at least a little bit. So I endorse your plan to visit an ER or urgent care clinic ASAP white also contacting your partner if able. I'll be interested to hear how it turns out. Please return after you have been seen and tell me how it goes. Good luck!

HHH, MD