For no apparent reason, I developed a very large, fluid filled blister across the top of one foot. It started out small and involved my second toe, but spread over a period of a few days until the top of my foot was involved to a diameter of approximately 3 inches. After about a week, it burst, and now the skin under it is red and quite tender. I am applying polysporin ointment and keeping it covered with sterile gauze. What could have caused this (not from shoe irritation) and is it likely to recur? I am female, 5ft 2in, 150 lbs, and in good health, except for hypertension for which I take Metropolol.