In her poem "After Great Pain, a Formal Feeling Comes," I agree that Dickinson leaves out the reason for the pain because it is not important to the poem. The poem focuses on the feeling of numbness—a "formal" [stiff] feeling that descends and enshrouds after the first grief of a painful event dissipates and depression sets in.
After that, the depressed feeling that Dickinson describes is one of emotionlessness, of being in state of shock that cuts off all feeling. At this point, whatever precipitated the pain doesn't matter as much as what emotions the sufferer is left with.
The nerves have gone dead, as if in a "tomb." It doesn't seem to matter when the event occurred, whether it be yesterday or a hundred years ago, because the speaker has emotionally shut down and cut herself off from it.
In this state of complete dull, emotionless depression, her feet, referred to not as her own feet, but as "the" feet—an object she is divorced from—go on moving, but she feels no part of them. It is as if she herself is not there, her physical body merely a mechanism. The calm or content she seems to exude really means that she has cut off her emotions and become as if she is a stone, a piece of quartz. She calls this time the "Hour of Lead."
The speaker goes on to state that this period of depression will end—is "outlived"—but she remembers it as feeling like a "Chill" and a "Stupor" before she lets it go.
By not revealing what caused the depression she has experienced, the speaker allows readers to apply the feeling she describes to their own painful situation, thus universalizing it.