Gallbladder disease (cholecystitis or gallstones): The severe left shoulder pain could be referred pain from the gallbladder, which is often felt in the right shoulder but can sometimes be referred to the left. Nausea is a common symptom, and vomiting can temporarily relieve the pain by reducing the pressure in the gallbladder.

Gastroesophageal reflux disease (GERD): Severe GERD can cause nausea and pain that might be referred to the shoulder. Vomiting could relieve the pain by emptying the stomach and reducing acid reflux.
Peptic ulcer disease: Similar to GERD, a peptic ulcer can cause severe abdominal pain that might be referred to the shoulder, along with nausea. Vomiting could provide temporary relief by reducing stomach acid.
Pancreatitis: While pancreatitis typically causes mid-back pain, it can sometimes be felt in the shoulder. Nausea and vomiting are common, and the pain might be relieved temporarily by vomiting, although this is less characteristic.

Myocardial infarction (heart attack): Although less common in a 30-year-old female, myocardial infarction can present with atypical symptoms, including shoulder pain and nausea. It's crucial to consider this diagnosis, especially if there are risk factors or associated symptoms like shortness of breath or arm pain.
Pulmonary embolism: A pulmonary embolism can cause sudden onset of shoulder pain due to referred pain from the lungs, along with nausea. While vomiting might not typically relieve the pain, it's a diagnosis that must be considered due to its high mortality rate if missed.
Aortic dissection: This is a rare but life-threatening condition where the inner layer of the aorta tears. It can cause severe, sudden pain that might be referred to the shoulder, along with nausea. Vomiting is not typically associated with relief of pain in this condition.

Splenetic infarction or rupture: These conditions can cause severe left upper quadrant pain that might be referred to the left shoulder, along with nausea. They are less common and usually associated with specific risk factors like sickle cell disease or trauma.
Diaphragmatic irritation (e.g., from a subphrenic abscess): This can cause shoulder pain due to irritation of the diaphragm, which shares nerve roots with the shoulder. Nausea and vomiting might be present, but this diagnosis is less common and usually associated with other symptoms or a history of abdominal surgery or infection.