It's not 'precum' you should be worried about, it's the actual left-over semen from your previous ejaculation. While you were in your gf, you could very well have been depositing the remaining semen in her.
This is stuff is very capable of getting her pregnant so that's the first problem.
The second problem is that she took the Plan B late (past the recommended time).
The third problem is that it sounds like the event happened at the worst possible time in her cycle.
So I'd say there is definitely a chance she could get pregnant from this. Not as high as if you two had just had normal sex with no Plan B at all, but still a chance.
All you can do now is wait for her period to show up. Good luck