I had mi a year ago and just been on long haul flight and have pain in leg from knee down which is especially bad at night ?

Might be thrombosis: You should see your doctor asap. You might have a deep venous clot or thrombosis (dvt). This is usually accompanied by pain, swelling and discoloration in the leg. Left untreated, DVT can cause lifelong complications in the leg and can lead to blood clots to the lung (pumonary embolism). These pulmonary emboli can be fatal. Please check with your doctor who can order an ultrasound test to check.