When Pinkie reunites with Sombra in Canterlot after going on a mission for the princesses, she's surprisingly distant with him. Not sure what to make of her behavior, Sombra tries to figure out the reason behind her actions.

It isn't until a discussion with her friends does he realize why she was so upset.

This is supposed to correlate with the "Reflections" arc in the MLP comics, taking place comics 17-20. This was written after 18's release and is correct with the information up to 19, which has not been released as of now.

Also, side story to this fic here:
The Not-So-Normal Adventures of King Sombra and Pinkie Pie

If you wish to read the comics, here are the available links for the arcs involved.

MLP: FIM Comic #17
MLP: FIM Comic #18