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This sounds like it could be a boil. I had one a few years ago in the exact same place. They're usually caused by a hair follicle that grows inward or becomes infected. The one I had got fairly large -- about the size of a quarter, and I finally went to the Dr. to have it checked out. He was able to "pop" it and drain the pus. Most boils don't have to be seen by the doc, but ones in your crotch area can be a special problem because of the sensitive area. You can try to bring the boil to a head yourself with hot compresses, but if the skin around the boil gets real red, painful and hot, you probably should see a doc. WebMD has a good page on boils and their home treatment along with precautions. (See source link below) Hope it gets better for you!

It is probably a tumor and you need to see a doctor because it could be cancer. Good luck.