Hello!
I have been through your question.
Related your concern I think first it is better if possible that you should be sure that your sexual partner is HIV positive or neg ( if possible ask him ). You should know that unprotected vaginal intercourse is a highly efficient route of HIV transmission because high concentrations of HIV can occur in semen and vaginal fluids , and because the genital tissues are very susceptible to infection.Based on your history (insertion of the penis for few seconds in vagina) you should explain me better if there was contact with sperm or not, because if there was not contact with semen during this short sexual act exist a small chance of been infected by HIV if you are women, but as well you should know that wherever a man’s penis is inserted it can cause small tears that make HIV infection more likely. Also if you are a man you have to be sure if you don't have any open sores in penis because the contact with HIV-infected blood or vaginal fluids could raise the risk of infection by HIV (even in a short contact ). But, if you are not sure related above mentioned questions and there is more likely that your sexual partner to be HIV positive, I suggest to talk with a HIV/AIDS specialist and to start as soon as possible the PEP.To be safe, assume that your sex partners are infected with HIV and please next time use condoms during of any kind sexual intercours, for your good.
I hope my answer help you.