Musculoskeletal strain or injury: Given the acute onset of pain upon standing and its exacerbation by ambulation, particularly when ascending or descending stairs, a musculoskeletal issue such as a strain or injury to the muscles or tendons in the lower back or abdominal wall is a plausible explanation. The patient's history of COPD could contribute to deconditioning, making her more susceptible to such injuries.

Biliary colic or cholecystitis: Although the pain is in the right lower quadrant, biliary issues can sometimes present with pain in this area, especially if the gallbladder is inflamed or a stone is obstructing the cystic duct. The absence of fever and jaundice does not rule out these conditions entirely.
Appendicitis: While the classic presentation of appendicitis includes fever, nausea, and pain that initially is diffuse or around the navel and then localizes to the right lower quadrant, atypical presentations can occur, especially in older adults. The severe pain and its persistence are concerning for appendicitis.
Diverticulitis: Inflammation of a diverticulum in the colon can cause severe pain, typically in the left lower quadrant but can occasionally be in the right lower quadrant if the diverticula are located in the right colon. The absence of changes in bowel habits or fever does not exclude this diagnosis.

Ovarian torsion: Although less common, ovarian torsion is a surgical emergency that can present with acute, severe lower abdominal pain. It's crucial to consider this diagnosis, especially given the acute onset and severity of pain, despite the patient's age.
Ectopic pregnancy: Extremely unlikely given the patient's age but must be considered in any female patient of childbearing age with acute abdominal pain, as it is a life-threatening condition.
Ruptured abdominal aortic aneurysm (AAA): While the patient is not on blood thinners and there's no mention of a known AAA, this condition can present with severe abdominal or back pain and is a medical emergency.
Intestinal obstruction: This can cause severe abdominal pain, nausea, and vomiting. The absence of vomiting or significant abdominal distension does not rule out a partial obstruction.

Inflammatory bowel disease (IBD) flare: Conditions like Crohn's disease or ulcerative colitis can cause severe abdominal pain but usually are accompanied by changes in bowel habits, which this patient does not report.
Right lower quadrant tumor: Either a primary tumor of the bowel or a metastasis could cause pain in this area, but this would typically be a more chronic process.
Psoas abscess: An abscess in the psoas muscle can cause lower abdominal or back pain and is often associated with fever and other signs of infection, which are not present in this case.