What Causes Blisters And Swelling In Legs?

my grandmother has a blister the size of a cantalope on her right leg. It broke, clear liquid oozed for two weeks, no infection. Recentlyl, the other leg got a blister the size of a dime. By the morning, turned to the size of a cantalope and full of water. E.R. Dr. broke it open and drained it. no infection. legs are swollen from knees down, retaining fluid. Causes? will it keep happening?