It seems you think that the condom slipped off while you had intercourse but that seems unlikely. Are you sure you didn't lose the condom after you pulled out and were too soft for it to stay on while you pulled out?
Only the head needed to be protected, so that is the only factor that you need to figure out if it happened. Likely she always uses a condom to protect herself so would be negative, however this is just a guess.
There is no statistic that can predict if you have HIV though, so you will have to determine if this happened, otherwise a test is the only thing to do because you can't let this hang over you forever.