It's common for cysts to form down in your groin area due to the amount of heat, pressure, and lack of "fresh" air down there. This doesn't sound like an STD at all.
Keep it clean and dry (use a cornstarch powder). I recently saw a product come on the market for chaffing that may be helpful. (For the life of me I cannot remember the name, but, the manufacturer of this product also makes other "feminine" products - so, you can look for it in the feminine needs aisle at your market).
The change in the shape of the cyst indicates that there could possibly be more damage below the surface of your skin, which would indicate the need for you to see your physician. It is likely that you will be prescribed an antibiotic (like Penicillin), and perhaps a topical ointment to help clear up the infection.
In the future, just be sure to keep the area clean and dry, apply powders as needed (especially during the summer months when you are more active and likely to sweat in the folds of your skin). Try not to wear clothes that are too tight against your skin (not only to prevent getting cysts, but, also yeast infections). If you can, switch to 100% cotton underwear under the problem goes away.
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