Thank you for your query
may be 1. due to constant irritation from the shoe that you are wearing. 2. It can be allergic (less likely).
If it is due to irritation it may help to keep an extra layer of padding in form of cotton between the ankle and the boot. But otherwise if it is very severe you should consult your orthopedic surgeon. He may make an alternative arrangement till that rash heals. For the rash itself, apply some antibiotic ointment which will also keep it lubricated. Maintain hygiene. You can keep your feet soaked in warm water for 5-10 min three to four times a day and then clean it dry with a clean towel. This will help speed up the recovery
Hope that helps
God bless you