Usually avascular necrosis is due to occlusion of arteries. Persistent elevation of factor VIII (without elevation of CRP) is a one of the risk factor for thrombosis (mainly venous). Pulmonary embolism is usually due to thrombus dislodging from lower limb. Hereditary hypercoagulable state (elevated factor VIII is one of them) is usually cause of this form of thrombosis. Birth control pills are acquired factor which causes thrombosis (risk increases in presence of hereditary factors - like elevated factor VIII in your case).
You will require detail work up for hypercoagulable state and may require lifelong anti-coagulation.