I have intense hip, lower back and side leg pain on the left side only when sitting or lying down. Your advice is appreciated.?

Hard to say. There are multiple potential causes, but the two most common is either a sciatica or a tendinitis on the side of the hip. The tendinitis condition is sometimes referred to as iliotibial band syndrome, tensor fascia lat a syndrome or as a bursitis of the hip and often responds well to a cortisone injection if lesser interventions fail. Thankyou for your question.