I have a slightly different perspective. I do not think you'll get HIV from her and would stop taking pep immediately. It's a toxic drug and is reserved typically for an exposure with a known HIV positive person. This person is not. So, then it is the same risk as all one time encounters of unprotected vaginal which is less than 1 percent. So, at 28 days and beyond, simply test with a 4th generation DUO test to be sure. But get off pep, it's not necessary.
What do you mean "slipped"? Only the head needs protection, and you said you took it off after so it sounds like it didn't slip off. You can't get hiv when you use a condom, so it is not surprising that she tested negative. Not sure why you took PEP after she got a negative result.