Welcome to the forum and thanks for your question.
It's quite common for the penis to slip out from a condom that is retained in the vagina. It's one of the reasons that all advice for proper condom use recommends withdrawing while still erect. However, this is still considered safe sex, because there is little if any contact of the penis with internal vaginal fluids.
In addition, assuming you are in the US or other industrialized country, it is statistically unlikely your partner had HIV. Even among the most sexually active heterosexual populations in such countries, fewer than 1 person in 1,000 has HIV. And even with no condom at all, if a woman has HIV, the chance of transmission to a male partner averages once for every 2,000 vaginal sex exposures. Your risk obviously would be even much lower than this.
Therefore, I don't even recommend HIV testing after this event, and certianly don't recommend you seek PEP. You really shouldn't be at all worried -- no more than you would have been had your penis not slipped out.
I hope this has helped. Best wishes-- HHH, MD