A 34-yeаr оld wоmаn whо is 4 hours post-pаrtum complains of sudden onset of left lower extremity pain. Clinical history reveals she has intermittently experienced left lower extremity pain and swelling for the past 7 years. Left lower extremity venous Doppler reveals fully compressible veins and no intraluminal echogenic matter with an adequate color Doppler signal. On pulsed wave Doppler, proximal compressions fully halt blood flow and distal compression demonstrates a good augment. However, Doppler reveals a dampened waveform down the entire left lower extremity. What is the most likely diagnosis?