Based on the context, this seems to be some catastrophic event in the not too distant past witnessed by Kaguya. It's probably a decent guess that we are looking at the firebombing of Tokyo in WWII (especially with the artist's emphasis on the embers and flames), but in the original work the atomic bombings of Hiroshima and Nagasaki can't be ruled out either. There's of course a few ways to interpret Kaguya's expression. What is certain is that, after waiting nearly 8000 years, she suddenly isn't sure she'll see Iroha again.
It's difficult to determine whether she thinks her presence has changed the time enough for a localized destructive event to make it impossible, or whether she thinks she is witnessing some end-of-the-world scenario (or whether it's just general despair at the state of humanity at this point in time).