Last summer after sitting outside on a summer night I noticed pinpoint sized blisters forming a line on my upper hip on the back side. It became itchy and swollen forming more blisters and was bright red all around. It eventually went away but it has came back maybe like 3 times since last summer in the same spot but not neccessarily in the same pattern. It has just came back and the blisters are in a cluster a little smaller than the size of a dime but the redness and swelling is about 3 times as big. Please, any ideas on what this is? Thanks.