Several things. Low blood volume due to bleeding, dehydration. Medications used for cardiac problems, hypertension. Quick changes in position going from sitting/lying position to standing, especially in the elderly varicose veins can cause leg swelling, burning, stinging, sharp pain, itching, restless legs symptoms, cramping, and is not a frequent cause of orthostatic hypotension.
Not really. Although many things can cause orthostatic hypotension, one thing that proably never causes it is varicose veins. On rare occasion, patients have been known to become dizzy on standing (but without orthostaic hypotension), when they have varicose veins, and then have the dizziness go away when their veins are treated. If you have orthostatic hypotension, your veins aren't likely to be the cause.
Get ultrasound. Venous doppler scan of legs.