Dr says she thinks I have bursitis in hip due to constant pain on left side. What causes this and what's best cure?

Arhritus. Degeneration of tissue and joint, trauma, even ischemia, and occasionally tumors. Find the cause, treat the discomfort, and the cause. Commonly goes to joint replacement in worst cases.
Inflammation. The bursa in the hip is like a cushion that allows the it band to glide on the outside of the hip (greater trochanter). This bursa may become inflamed (bursitis) if the muscles that help to move the hip (the abductors) are weak and don't support the it band as well as they should. Strengthening these muscles with pt and decreasing the inflammation with nsaids is the best first treatment.
Iliotibial band. Trochanteric bursitis is an inflammation of the space between the iliotibial band and the prominent part of your upper femur called the greater trochanter. When the tendon rubs against the femur, inflammation results. The treatment involves reducing the inflammation and lessening the rubbing. Ice, nsaids, allow with stretching the it band are a start. Occasionally steroids are injected .