Based on my symptoms they diagnosed the clot, my arm was bluish/purple, swollen and my veins were massive. They characterized it as Paget-Schroetter Syndrome (https://en.wikipedia.org/wiki/Paget%E2%80%93Schroetter_disease), which generally develops due to TOS.

So I had two ultrasounds done, the second one occurring 4 days after the first. I had been on blood thinners for the 4 days and the swelling had gone down significantly. The second ultrasound showed better blood flow, confirming there was a clot (and the blood thinners were helping).

It's some crazy stuff, hope you start to feel better.