Rib Fracture: Given the patient's mechanism of injury (falling onto her left side) and the location of her pain, a rib fracture is the most likely diagnosis. The patient's age and osteoporosis risk (due to age and possibly long-term steroid use for seizure disorder, though not explicitly mentioned) increase her susceptibility to fractures.

Musculoskeletal Injury: The patient could have sustained a musculoskeletal injury (e.g., bruising, strain) to the left side of her body, including her ribs and abdominal muscles, from the fall.
Abdominal Wall Contusion: A contusion (bruise) of the abdominal wall could also explain her pain, especially if she landed directly on her abdomen or side.
Costochondritis: This is an inflammation of the cartilage that connects the ribs to the breastbone and could be exacerbated or caused by the fall.

Spleen Injury/Laceration: Although the patient denies nausea, vomiting, or headache, and there's no mention of anticoagulant use, spleen injury is a critical diagnosis to consider due to the potential for severe internal bleeding. The absence of these symptoms does not rule out a spleen injury.
Pneumothorax: A pneumothorax (collapsed lung) could occur if a rib fracture punctured the lung. This is a medical emergency and must be considered, especially given the mechanism of injury and the patient's significant pain.
Hemorrhage: Internal bleeding could be occurring without overt symptoms, especially in the elderly, who may not always present with typical symptoms of shock or severe blood loss.

Osteonecrosis: Although rare, osteonecrosis (death of bone tissue due to lack of blood supply) could potentially occur, especially if the patient has a history of steroid use for her seizure disorder.
Pathological Fracture: If the patient has an underlying bone disease (e.g., metastatic cancer, osteomyelitis), a pathological fracture could have occurred with minimal trauma. However, there's no specific information in the history to suggest this.