The only risks for HIV in adults are:
1) Having unprotected anal or vaginal sex, or
2) sharing intravenous needles with IV drug users.
You cannot get HIV or any STD through clothing, including underwear, even if the clothing has blood or fluid on it.
If you do not engage in any of the activities I mentioned above with someone HIV+ or whose HIV status is unknown to you, you will NEVER have to worry about getting HIV. There is no information you could add about this event that would make it a risk for HIV.