No one here can figure how long it was, but any unprotected anal is a risk. On the other hand, you don't always get hiv when the other person is positive. If she always uses a condom she will be negative. Did she supply or ask for the condom?
You aren't likely AT ALL to get HIV from 5 seconds of unprotected penetration.
For full-on unprotected anal sex, the risk for the top is 0.11% if circumcised, and 0.62 if uncircumcised. These numbers translate to a 99.89% and 99.38% chance of NOT getting HIV. PEP would make the odds of getting HIV astronomical.
But again, I wouldn't freak out at all over 5 seconds of unprotected penetration.