Since you have blisters spreading you better get a dermatologist opinion and skin biopsy
done to know the plane of blisters and completebloodcount,ESR,liver function test and bloodsugar level,Chestxray,ECG,Ultrasound
abdomen done to track the cause.It could probably be InsectBite Reaction or even Vasculitis
.Consult a dermatologist
as soon as possible.