hi i had gone through your query and understand your concerns. i would come up with two possibilities.
1.the first one is, SUN ALLERGY; exposure to ultraviolet rays of the sun is the triggering factor your sun burn and it is related to inherited tendency.
2.second possibility is, VITILIGO;
-abnormal auto immune response
i advise you taking Homeopathic
medicines gives safe and permanent cure for your complaints
consult your local homeopathic physician for correct diagnosis of the case and remedy to fit your complaints
I hope this is helpful for you, thank you