That passage is from the flash fiction story Note To Self by Tracy Guzeman. The protagonist makes a brief list of life memories, those she would chose to forget and those she would chose to remember.
She would chose to forget the memory of the look on her son's face, presumably this is one of the most painful memories in her life - she has chosen to accuse her son without proof, and has hurt him deeply, which she regrets. My interpretation of "crack open" is that the son's face reveals the pain and perhaps change in perception about his mother, and "your world shifted" is the effect this has on the mother, perhaps a change in perception about who she is, or what is happening to her. I won't spoil the story in case you wish to read it, being flash fiction it is very brief.
Of note is the following in Wikipedia's article about flash fiction, emphasis mine:
Unlike a vignette, flash fiction often contains the classic story
elements: protagonist, conflict, obstacles or complications, and
resolution. However, unlike a traditional short story, the limited
word length often forces some of these elements to remain unwritten –
that is, hinted at or implied in the written storyline. Different
readers thus may have different interpretations.