Musculoskeletal strain or overuse: This is the most likely diagnosis given the symptoms of leg heaviness and pain that are exacerbated by movement, such as changing sides, sitting in a car, and getting up. These activities can strain the muscles and joints in the legs, leading to pain and discomfort.

Venous insufficiency: This condition, where the veins have problems returning blood from the legs to the heart, can cause leg heaviness and pain, especially after periods of inactivity or when changing positions.
Peripheral artery disease (PAD): PAD involves the narrowing of the arteries that supply blood to the legs, which can lead to pain and heaviness in the legs, particularly when walking or changing positions.
Lumbar radiculopathy or sciatica: Compression or irritation of the nerves in the lower back can cause pain, numbness, and heaviness in the legs, which may worsen with certain movements.

Deep vein thrombosis (DVT): Although less common, DVT is a potentially life-threatening condition that can cause leg pain and swelling. It's crucial to consider DVT, especially if there's a history of recent immobility, surgery, or cancer.
Pulmonary embolism (if symptoms suggest): While not directly related to leg heaviness and pain, if there are accompanying symptoms like shortness of breath or chest pain, pulmonary embolism must be considered as it can be fatal.
Spinal stenosis or cauda equina syndrome: These conditions involve compression of the spinal cord or nerves and can cause severe leg pain, numbness, and weakness. They are medical emergencies requiring prompt intervention.

Chronic compartment syndrome: This condition involves increased pressure within the muscle compartments of the legs, leading to pain and swelling, typically after exercise.
Neurological disorders (e.g., multiple sclerosis, peripheral neuropathy): Certain neurological conditions can cause leg pain, heaviness, or weakness, though these would typically be accompanied by other neurological symptoms.
Vasculitis or other autoimmune diseases: Conditions like vasculitis can cause inflammation of the blood vessels, leading to leg pain and other systemic symptoms, but these are less common and usually present with additional symptoms.