Heartburn is commonly used interchangeably with gastroesophageal reflux disease (GERD) rather than just to describe a symptom of burning in ones chest. Differential diagnosis
Heartburn-like symptoms and/or lower chest or upper abdomen may be indicative of much more sinister and/or deadly disease. Of greatest concern is to confuse heartburn (generally related to the esophagus) with a heart attack as these organs share a common nerve supply. Numerous abdominal and thoracic organs are present in that region of the body. Many different organ systems might explain the discomfort called heartburn. Heart
The most common symptom for a heart attack is chest pain. However, as many as 30% of chest pain patients undergoing cardiac catheterization have findings that do not account for their chest discomfort. These are often defined as having "atypical chest pain" or chest pain of undetermined origin. Women experiencing heart attacks may also deny classic signs and symptoms and instead complain of GI symptoms. One article estimates that ischemic heart disease may appear to be GERD in 0.6% of people. Esophagus
GERD (most common cause of heartburn) occurs when acid refluxes from the stomach and inflames the esophagus. Esophageal spasms typically occur after eating or drinking and may be combined with difficulty swallowing.
Myalgia (also called muscle pain and muscle ache in laymans terms) is the medical term for muscle pain. Myalgia is a symptom of many diseases. The most common cause of acute myalgia is the overuse of a muscle or group of muscles; another likely cause is viral infection, especially when there has been no trauma. Long-lasting myalgia can be caused by metabolic myopathy, some nutritional deficiencies, and chronic fatigue syndrome. Causes
The most common causes of myalgia are overuse, injury, and strain. Myalgia might also be caused by allergies, diseases, medications, or as a response to a vaccination. Dehydration at times results in muscle pain as well, especially for people involved in extensive physical activities such as workout. Muscle pain is also a common symptom in a variety of diseases, including infectious diseases, such as influenza, muscle abscesses, Lyme disease, malaria, trichinosis or poliomyelitis; autoimmune diseases, such as celiac disease, systemic lupus erythematosus, Sjögrens syndrome or polymyositis; gastrointestinal diseases, such as non-celiac gluten sensitivity (which can also occur without digestive symptoms) and inflammatory bowel disease (including Crohns disease and ulcerative colitis).The most common causes are:

Overuse
Overuse of a muscle is using it too much, too soon or too often. One example is repetitive strain injury. See also:

Exercise
Weight lifting

Injury
The most common causes of myalgia by injury are: sprains and strains.