A small pain in my chest is a very nice and emotional poem about a young soldier who is shot in his chest, but he was shot at the time when an underground mine is given in the poem that the young soldier asks his colleague or his senior for some water and then the young soldier tells the narrator that he is lucky enough because all the other soldiers are dead but he only had a small pain his chest.
by reading this line we understand that the shot soldier is unaware that he had been shot in his chest as he says there is only a small pain in his chest.
in the end  we come to know that the young soldier is dead and the narrator(the person who gave him some water) has a severe wound in his heart of seeing so many of his colleague dead than the young soldier who just had a small pain his chest