Dehydration: Given her history of dehydration and current symptoms of leg cramping and heavy legs, dehydration is a likely cause. Dehydration can lead to electrolyte imbalances, which can cause muscle cramping.

Electrolyte Imbalance: Her history of dehydration and current symptoms suggest an electrolyte imbalance, such as hypokalemia or hypocalcemia, which can cause muscle cramping and weakness.
Medication Side Effect (Prednisone): Long-term use of prednisone can lead to muscle weakness and osteoporosis, which may contribute to her symptoms of heavy legs and difficulty standing.
Peripheral Artery Disease (PAD): Intermittent claudication, characterized by cramping in the calf area, is a symptom of PAD, which is a possible diagnosis given her age and symptoms.

Deep Vein Thrombosis (DVT): Although less likely, DVT is a potentially life-threatening condition that can cause leg swelling, pain, and heaviness. Given her history of interstitial lung disease and immobility, she may be at increased risk for DVT.
Cellulitis or Infection: Infection can cause leg pain, swelling, and redness, and can be life-threatening in elderly patients. A thorough examination and diagnostic workup are necessary to rule out infection.
Neurological Disorder (e.g., Neuropathy): Conditions like peripheral neuropathy can cause leg pain, weakness, and heaviness, and may be related to her interstitial lung disease or other underlying conditions.

Rhabdomyolysis: A rare condition characterized by muscle breakdown, which can cause muscle pain, weakness, and dark urine. Although unlikely, it is essential to consider this diagnosis, especially if she has a history of trauma or certain medications.
Vasculitis: Inflammation of the blood vessels can cause leg pain, weakness, and heaviness, but is a rare condition that would require further diagnostic testing to confirm.