Chances that Nene fell out of love with Amane for “no real reason” = [ Maybe more than 0, but vanishingly small.]
Sure, things “just happen that way” in real life, but they rarely do in stories, and I don’t get the sense that this one is going for gritty everyday realism.
Stories are about reasons, and when one is withheld like the reason for that breakup (given the story’s focus on the former-ness of the Nene-Amane romantic relationship), it’s highly unlikely to turn out to be that Nene just realized the day before graduation that she wasn’t all that into Amane anymore.
My money is on something else going on with Nene that we know very little about as of yet.