Musculoskeletal strain or costochondritis: The patient's symptoms of intermittent pain along the lower rib cage, exacerbated by movement from bending to standing, suggest a musculoskeletal origin. Costochondritis, an inflammation of the cartilage that connects the ribs to the sternum, is a common cause of chest wall pain and could be triggered by the described movements.

Gallbladder disease: The location of the pain (right lower rib cage) could be referred pain from gallbladder pathology, such as cholecystitis or gallstones. The pain pattern (on and off for two years) might suggest intermittent gallbladder dysfunction.
Peptic ulcer disease: Although the pain is on the right side, peptic ulcer disease can sometimes cause referred pain to the lower rib cage area, especially if the ulcer is located in the duodenum.
Liver disease: Conditions like hepatitis or liver cirrhosis could cause right upper quadrant pain that might be perceived as lower rib cage pain, depending on the patient's description and the examiner's interpretation.

Pulmonary embolism: Although less likely given the chronic nature of the symptoms, pulmonary embolism can present with sudden onset of chest pain that worsens with deep breathing or movement. It's crucial to consider this diagnosis due to its high mortality rate if missed.
Myocardial infarction: Chest pain is a classic symptom of myocardial infarction. While the location and nature of the pain might not be typical, atypical presentations are more common in older adults, making it essential not to miss this potentially fatal condition.
Pneumonia: Lower lobe pneumonias can cause pain that is perceived in the lower rib cage area, especially if the patient has pleurisy (inflammation of the lining surrounding the lungs).

Tietze's syndrome: A rare inflammatory condition of the cartilages and surrounding tissues of the ribs, similar to costochondritis but can involve any part of the chest wall.
Sickle cell crisis: In patients with sickle cell disease, a crisis can cause severe pain episodes, including chest pain, due to vaso-occlusion.
Osteomyelitis or rib fracture: Although less common, osteomyelitis (infection of the bone) or a stress fracture of the ribs could cause localized pain that worsens with movement.