I seems that you have had an episode of cellulitis
and the inflammation is still continuing. As there is no evidence of infection in the blood and the scan that you have done shows no clot, you donot have any infection or deep vein thrombosis
. But as you have mentioned that you had blocked capillaries 20 years back, the swelling could probably because of the damaged venous system which might have aggravated recently due to some self limiting infection or trauma. You can try keeping your leg elevated on two pillows when you are resting to reduce the swelling and pain. You could visit a Surgeon for a check up.