IINM, this is from Fate/Hollow Ataraxia. More specifically the scene where Shirou and Rin get themselves trapped in a magical chest made by Zelretch and try to ask for help by phone. Doing so, they discover that the space inside of the chest is connected to an parallel world and they end contacting alternate versions of Taiga, Luvia and Rin herself.
I think this illustration is supposed to depict the latter who, unlike the regular Rin, completely dropped the "tsun" of her tsundere act and proves to be openly and shamelessly head-over-heels in love with Shirou.
His first person narration puts it the best (from the English patch):
...It was definitely Tohsaka's voice coming over the telephone, but something about it was inconceivable.
As though it was... lovestruck.
It reminded me of lovers who shamelessly embrace each other in public.
