Pain and swelling below the knee can occur due to causes such as a muscle strain, ligament injury, infection, fluid buildup, or conditions like Deep Vein Thrombosis or Baker's Cyst. If the swelling is persistent, painful, or associated with redness, warmth, or difficulty walking, it is important to consult a doctor for proper examination and tests such as an X-ray or ultrasound to find the exact cause and start appropriate treatment.