Septic arthritis or osteomyelitis: Given the patient's history of intravenous drug use (IVDU) and hepatitis C, she is at increased risk for infections, particularly those affecting the bones and joints. The inability to sit down or walk suggests significant pain and possible infection in the lower extremities or pelvic region.

Cellulitis or abscess: IVDU increases the risk of skin and soft tissue infections, which could cause significant pain and mobility issues if located in areas critical for walking or sitting, such as the buttocks or thighs.
Hepatic encephalopathy or other hepatic complications: Although less directly related to the inability to sit or walk, advanced liver disease from hepatitis C could lead to a range of systemic symptoms, including muscle weakness or cognitive impairments that might indirectly affect mobility.
Musculoskeletal injury: Direct trauma or overuse injuries could also cause significant pain and difficulty walking or sitting, especially if the patient has a history of poor health or malnutrition.

Spinal epidural abscess: A serious and potentially life-threatening condition that requires prompt diagnosis and treatment. Back pain, fever, and neurological deficits are common presentations, but early on, symptoms might be nonspecific, including difficulty walking due to pain or neurological impairment.
Osteonecrosis (avascular necrosis): Particularly of the femoral head, which could be related to chronic steroid use, excessive alcohol consumption, or possibly as a complication of hepatitis C treatment. It presents with hip pain that worsens with weight-bearing activities.
Neurological conditions (e.g., Guillain-Barré syndrome, transverse myelitis): Although less common, these conditions can present with rapid onset of weakness or paralysis and could be related to the patient's hepatitis C status or as a reaction to a recent infection.

Infective endocarditis with septic emboli: While the primary presentation involves cardiac symptoms, septic emboli can cause a wide range of systemic symptoms, including pain and mobility issues if they lodge in joints or muscles.
Bone or soft tissue tumors: Either primary or metastatic, could cause localized pain and mobility issues, though these would typically have a more gradual onset.
Vasculitis: Certain types, like giant cell arteritis or polyarteritis nodosa, could present with systemic symptoms including pain and weakness, though they are less common and might not directly cause an inability to sit or walk without other symptoms.