This is very ominous. The way Sumi is thinking and talking, the "almost too pale" thing, the thing with her on the phone with her father...
She'd better not be dying. If this switches from childhood friends sweet slice of life romance into 'The Summer You Were There'... I will never forgive the mangaka.
Even if Sumi is just I'll and has to move away, separating them, it's a needlessly melodramatic twist.