Very Bright Flash.

This just happened. Its 8:20 pm EST, Wednesday, November 27th. I am in the Library sitting at the computer. A very bright flash, almost blinding, just occurred out of the bookcase, immediately to my left and about 4 0r 5 foot from the floor. The day has been heavily overcast and it has been snowing ,off and on, all day. There are no windowds across from where the flash occurred. The only windows in the Library are two very tall, very slim, gothic type and they are directly behind me. When I looked in the direction of the flash, once it was over, there was nothing to see but books crowding the library shelves. There is nothing to be seen, out the two windows but evergreen trees and a couple of pear trees. View of the road in front of the house is blocked by two huge old evergreens and the angle is all wrong even if those trees were not there. What the heck was it? This is weird. The flash was just like someone had taken a flash pictuire of me sitting at the computer. No one was there and the book shelves in that area of the bookcase are tightly packed and there is a 17 inch solid brick wall behind the bookcase. Any suggestions?