Costochondritis: This condition, also known as Tietze's syndrome, is characterized by inflammation of the cartilage that connects the ribs to the breastbone (sternum), which could explain the sharp pain underneath the right pectoralis major and shoulder area without exacerbation by movement. The patient's history of diabetes and hyperlipidemia (HLD) may contribute to an increased risk of developing musculoskeletal issues.

Musculoskeletal Strain: Given the location of the pain, a strain in the muscles of the shoulder or chest wall could be a plausible explanation. The absence of a known injury does not rule out this possibility, as minor, unnoticed strains can occur.
Gallbladder Disease: The patient's history of HLD increases the risk for gallstones, which can cause intermittent, sharp pain in the right shoulder or chest area due to referred pain.
Peptic Ulcer Disease: Although less common, peptic ulcers can cause chest pain that may be referred to the shoulder area. The patient's diabetes and HLD might contribute to an increased risk of gastrointestinal issues.

Myocardial Infarction (MI): Although the patient denies shortness of breath or worsening pain with deep breathing, and there's no mention of exertional exacerbation, cardiac causes of chest pain must always be considered, especially in a patient with risk factors like diabetes and HLD. Atypical presentations of MI can occur.
Pulmonary Embolism (PE): The absence of shortness of breath or worsening pain with deep breathing does not entirely rule out PE, as some patients may present atypically. The sharp, intermittent nature of the pain could potentially be related to a pulmonary embolism, especially if there are other underlying risk factors not mentioned.
Pneumothorax: A spontaneous pneumothorax could cause sharp chest pain and is a critical condition that requires immediate diagnosis and treatment.

Thoracic Outlet Syndrome: This condition involves compression of the nerves and/or blood vessels that pass into the thoracic outlet (the space between your collarbone and first rib). It could potentially cause sharp, intermittent pain in the shoulder and chest area but is less common.
Pancreatitis: Although the pain profile is typically more epigastric, radiating to the back, pancreatitis could be considered, especially if there are other gastrointestinal symptoms or risk factors for pancreatic disease.

To evaluate the patient, the following diagnostic studies could be considered:

Electrocardiogram (ECG): To rule out myocardial infarction or other cardiac issues.
Chest X-ray: To evaluate for pulmonary issues such as pneumothorax or pneumonia.
Blood Tests: Including complete blood count (CBC), blood chemistry tests (e.g., troponin for cardiac damage, liver enzymes for gallbladder disease), and inflammatory markers.
Ultrasound of the Gallbladder: If gallbladder disease is suspected.
CT Scan of the Chest: If pulmonary embolism or other thoracic issues are considered.
Musculoskeletal Ultrasound or MRI: If musculoskeletal strain or costochondritis is suspected and needs further evaluation.

Each diagnostic step should be guided by the patient's clinical presentation, risk factors, and the results of initial evaluations.