Costochondritis: Given the patient's description of sharp pain in the left posterior rib area radiating around the flank into the left upper abdominal region, costochondritis is a plausible diagnosis. This condition, which involves inflammation of the cartilage that connects the ribs to the breastbone, can cause sharp pains in the chest wall that may radiate to the abdomen.

Gastroesophageal Reflux Disease (GERD) exacerbation: The patient's history of GERD and current complaint of abdominal pain worsening over 48 hours could suggest an exacerbation of GERD, potentially causing referred pain to the left posterior rib area.
Irritable Bowel Syndrome (IBS) flare: Although IBS typically presents with abdominal pain and changes in bowel habits, the patient's history of IBS C (constipation-predominant) and current abdominal pain could indicate a flare of IBS symptoms.
Musculoskeletal strain: The patient's recent onset of sharp pain could be attributed to a musculoskeletal strain, possibly from coughing due to her asthma or from other physical activities.

Pulmonary embolism: Although the patient denies fever, chest pain, or shortness of breath, pulmonary embolism is a life-threatening condition that must be considered, especially given her recent start of a weight loss medication (Zepbound) which may increase the risk of thromboembolic events.
Pneumonia: Despite the lack of reported fever or respiratory symptoms, pneumonia can present atypically, especially in patients with underlying conditions like asthma, and must be ruled out.
Gallbladder disease: The patient's abdominal pain radiating to the left upper abdominal region could suggest gallbladder disease, including cholecystitis or cholelithiasis, which would require urgent evaluation.

Sickle cell crisis: Although less likely given the patient's demographic and lack of reported history of sickle cell disease, a sickle cell crisis could cause severe pain and must be considered in the differential diagnosis.
Pancreatitis: The patient's history of gastroparesis and current abdominal pain could suggest pancreatitis, although this would be less likely without other symptoms like nausea, vomiting, or elevated pancreatic enzymes.
Splenic infarction or rupture: Given the patient's left upper abdominal pain, a splenic infarction or rupture, although rare, must be considered, especially if there is a history of trauma or underlying conditions that may affect the spleen.