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There are many reasons for the symptoms that you are describing,
STIs are on the top of the list, and certainly need to be considered,
Among the Non STIs Contact dermatitis due to the sanitary pad is certainly an area of concern,
I would recommend changing the brand, or NOT using pads for a few days, applying vaseline on outer area, and taking oral anti allergy medicine like Zyrtec, and consulting an ob/gyn if this does not help.
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