Hi
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It could be allergy or chicken pox or measles. All three can be preceded by flu like symptoms. Please consult your doctor who can examine the blisters and treat it. Take care!
I am sorry I don't have the answer. What you should do though is go to a doctor. It could be something small, but there is no reason to chance it, and since it is so quickly you noticed it, if it was something big, you are catching it in really good time.