Itchy Heel

I have incredible itching sensations in my left heel.
For the longest time it has been this way and only in my left heel.
Sometimes I feel as if I could scratch it with a knife.
The deeper the better it feels.
What could be the cause of this.
I am a large frame male and could be considered overweight.
However I am just a large frame man.
I work out 5 days a week and do plenty of cardio.
What do you think??