You have not mentioned which shoulder it is, hence I’ll explain regarding both one by one.
Pain under the left shoulder blade is usually a
referred pain from abdomen like stomach ulcers, pancreatitis, or ectopic pregnancy (unlikely in her case but one has to be sure) or from the chest like in heart attack, pericarditis, pleuritis, pneumonia, pulmonary embolus, and aortic dissection.
Pain under right shoulder blade is usually due to gall stones, duodenal ulcer and liver infection.