Musculoskeletal strain or osteoarthritis: Given the description of dull pain in the groin area and the absence of red flag symptoms such as fever, vomiting, or significant weight loss, a musculoskeletal issue is a plausible explanation. The patient's age and the nature of the pain support this diagnosis, as osteoarthritis and musculoskeletal strains are common in this demographic.

Hernia: Although the patient had a recent CT abdomen and pelvis that was negative, a hernia could still be a possibility, especially if it's small or not easily visible on imaging. Groin pain is a common symptom of hernias.
Diverticulitis or other gastrointestinal issues: Despite the previous negative CT, diverticulitis or other gastrointestinal issues could still be considered, especially if the patient's symptoms have changed or if there were limitations to the previous imaging.
Urinary tract issues: Kidney stones or other urinary tract problems could cause groin pain, especially if the pain radiates from the back or abdomen to the groin.

Aortic dissection or aneurysm: Although less likely given the patient's presentation and previous negative imaging, aortic dissection or aneurysm is a critical diagnosis that cannot be missed due to its high mortality rate. The presence of nausea and groin pain could be indicative of a dissecting aneurysm, especially in an elderly patient.
Ovarian torsion or other gynecologic emergencies: In postmenopausal women, ovarian torsion is less common but still a possibility. Other gynecologic emergencies, such as ruptured ovarian cysts, could also present with acute groin pain.
Appendicitis: Although the pain is on the right side and the previous CT was negative, appendicitis can sometimes present atypically, especially in elderly patients. It's a diagnosis that should be considered due to its potential for serious complications if not treated promptly.

Psoas abscess: A collection of pus in the psoas muscle can cause groin pain and is more common in patients with underlying conditions such as spinal infections or Crohn's disease.
Retroperitoneal fibrosis: A rare condition characterized by the proliferation of fibrous tissue in the retroperitoneum, which can cause abdominal or groin pain due to the encasement of structures.
Neoplastic processes: Tumors in the retroperitoneal space or in the groin area could cause pain, although these would be less common and might be associated with other symptoms such as weight loss or a palpable mass.