Severe lower back pain with left leg pain that worsens on sitting is often due to nerve compression like Sciatica or a slipped disc. Avoid sitting on the floor or bending, use firm back support, apply warm compresses, and do gentle stretching exercises. For relief, you can take Paracetamol or, if suitable, an anti-inflammatory medicine. If pain is persistent, severe, or associated with numbness, weakness, or difficulty walking, consult a doctor for proper evaluation and treatment.