This study aims to unveil how COVID-19 affected the experience of stress by focusing on the stressors. Using computational analysis based on a newly developed stressor identification model, we compared the experience of stress expressed by Korean Twitter users before and during the pandemic in terms of (1) the stressors as the source of stress and (2) emotion as the manifestation of stress. Both tweet-level (N = 202,556) and user-level (N = 24,803) analyses revealed that social factors are prevalent sources of stress both before and during the pandemic. Moreover, social stressors increased the most during the pandemic. While stress from social stressors was manifested mainly as sadness before the pandemic, anger became the predominant emotional manifestation during the pandemic. Public health policies and educators should consider social stressors as the predominant source of stress during the pandemic and seek ways to prepare the public better for such threats.