I Recently Engaged In Rubbing With A Latin Transexual With

I recently engaged in rubbing with a latin transexual with unknown HIV status, she briefly rubbed her unprotected penis against my anus with lubrication on it but did not penetrate. I am not sure if she had pre seminal fluid on her penis or not. I took a Determine Combo ag/ab rapid test at 15 days, and it was negative. My questions are how great of a risk was this situation and how accurate are those results at 15 days? Thank you docs.