I wasn't sure first if I remembered the title right because the summary doesn't fit what I remembered / your discription but this is the story I was thinking of
*quote*
"...it can't be her..."
"...but she's the spitting image of the girl in the paintings...!"
"How can it be her...? They're over one hundred years old."
Past Present by jamesmarstersjunkieSumary:
I often thought of what would have happened had things been different in season two. The epi in which Buffy comes down with the flu and is taken to the hospital. Then I thought of what if there was a place our minds and selestial bodies go to if we get too sick. Time knows not the minds subconscience nor does it control how our bodies react to outside stimuli. Our minds are then free to roam to any plane it finds comfort in; maybe, then, if two people from different times find each other in this place their interactions could effect thier real worlds. And if that's possible they could then change the corse of the future. finished