In a thin person, the Intra Cardiac Defibrillator lead, occupies greater part of initial part of the vein and your sleeping on left side has led to the situation you are having. You do not need more than Ultrasound
to confirm or rule out the diagnosis of Deep Venous Thrombosis
. You are already taking anticoagulants therefore I infer you do suffer from arrhythmia
. What is needed is cause for recurrent episodic DVTs in right leg despite anticoagulants. I am certain your doctor will agree with me that either INR (international normalized ratio) is not being maintained which can be done with some diligence. Second, not that uncommon possibility is your blood needs to be tested for tendency for excessive clotting, the tests procoagulant activity and platelet function can be decided and done by a Hematologist
to whom your doctor refers you to. While sleeping keep a soft pillow below left arm and it should be slightly at higher level than rest of your body
In nut shell these come under minor complications of a major procedure and eminently curable thus I see cause for concern not for worry.
with best wishes.