Thank you for making contact and I can understand your concerns. It seems there are two issues here:
Firstly your concerns about blood clots, or what is medically known as deep vein thrombosis. Immobilisation can be a risk factor, together with others including a previous history of clots, being on a combined oral contraceptive, various auto-immune conditions and also if you smoke. Then there are the flight factors such as being in a cramped cabin, drinking alcohol and the flight duration. Indeed, there are many!