My Husband Has Very Large Swollen Welts On His Back

My husband has very large swollen welts on his back and leg. It is slightly itchy and very red. He does not remember being bitten by any insects. In fact it is so cold I can't imagine any insects around. He notice the first one last week while out of town. He noticed two more today. When I say large I'm talking about like 2"x2" oval shape. He is 61 yrs old and takes medicine for high blood pressure.