You've likely felt a burning sensation in your chest at some point, perhaps after eating too quickly or consuming something acidic. While heartburn can be a nuisance, it's usually harmless. However, there are times when it could signal something more serious. Heartburn, or acid reflux, occurs when stomach acid and sometimes partially digested food flow back up into the oesophagus. Your stomach is built to handle acid, but your oesophagus isn't, resulting in a burning and irritating sensation.