What Causes Red Hot Blister On The Lower Shin?

my aunt is 46 years old about 100 lbs over weight and has had prolonged swelling in her left leg for 1 1/2 years. meanwhile she had an itch and used her heal to scratch it and it broke her skin over 3 months ago since then it has become an ulcer on lower shin of left leg , its red, hot, and very painful around ulcer, and leg has become numb this last week on inside of leg. ultrasound was done and no clots in leg.