I agree. The main thing is that the head of your penis was covered as that is the only risk when they say unprotected sex. So, you had no risk. I'm glad you used protection. Keep doing that and don't share IV drug needles and you will not ever have to worry about HIV.
Condom slippage during withdrawal is common and it is not a risk since during the act urethra was protected and the only time the condom wasn't there on your penis was when your penis was outside of the vagina.
Also, she is your girlfriend, not sure of your circumstances! However, if it was my partner, I would have trusted her 100% and this event wouldn't really bother me.
If your situation is different, you have some thinking to do if not, forget about this event and move on because neither you were at risk nor you had a reason to doubt her.