It is most likely to be some form of allergy or dermatitis, and the commonest causes of this in this area is a reaction to something you are coming into contact with. This could be your new underwear which has some synthetic fibers in it and they could cause this reaction. Or it could be a vaginal deodorant or spray if you use one, or even a bath shampoo of some sort. All new materials or chemicals may cause some allergic reaction. So think about whether there is something you are coming into contact with in this region that might be causing it. Removing that factor makes the swelling disappear . Other possibilities are hormonal fluctuations, an infection, etc. The best advice would be to make an appointment with a health care provider for an evaluation.
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