There are a couple of stories where Buffy becomes pregnant and some where Spike thinks she cheated on him (i.e Live and Breathe by Mercy at Sinister Attraction, although it's based after Chosen). The only one that I can find however that has the bit where Buffy talks to a rep from the PTB is in Whispers of Eternity by Ashlee. You can find it at Elysian Fields and the summary is:-
Battling a demon on a routine patrol one night, Buffy is seriously injured. When Giles realizes the full extent of the damage, he knows there is only one person who can help her get back to the Slayer she used to be.
It is based in Season 5 so it may not be the one you're looking for, but it's a good one nonetheless.
Enjoy!

Sarah