How to manage knee pain caused by a long flight journey?

Hello doctor,

I recently traveled abroad and it was around 20 hours of flying both ways. I returned two days ago and went to the doctor with some pain in my knee. He ordered an ultrasound which came back negative for blood clots. Yesterday, I worked myself into a panic attack and returned to the doctor who then did a D-dimer. My results came below 0.5, specifically at 0.41. However, I had a D-dimer last year that was 0.33, so it is elevated based on that baseline. The doctor who delivered both results said to come back if the pain did not subside or got worse. As of today, the pain is not worse but does feel like it has spread into the top of my calf and back of my thigh. It feels kind of like a minor cramp. My question is whether it is possible that these tests were conducted too early, potentially leading to a misdiagnosis of DVT. And could my increased D-dimer levels be a result of the early stages of a blot clot? And if so, would you recommend I go to the ER tonight or wait until the doctor opens in the morning?

Thank you.