From the long duration, the following are the likely causes:
1. Chronic infections including secondary to an ear discharge
or a cold sore
2. Contact Dermatitis
. Do you wear spectacles?
3. Parasitic infection
. Which part of the world do you live in?
4. As a part of systemic disease or developing tumor. What is your age?
Get a skin specialist to examine you. Do not break it without antibiotic cover and precautions to prevent spread. The fluid from the blisters or a biopsy from the edge of the sore can be examined to determine the origin.