What does her lungs were on fire mean?

The phrase "her lungs were on fire" is a metaphorical expression that typically describes a feeling of intense pain or discomfort in the chest or respiratory system, often associated with strenuous physical activity, such as running or exercising. It conveys a sense of overwhelming exertion and struggle to breathe, suggesting that the person is pushing their limits. The imagery of fire emphasizes the intensity of the sensation, highlighting both physical strain and emotional turmoil.