If you're not sexually active AND when they itch, they itch alot, it sounds like ecxema. Ecxema is an auto-immune disorder. Lots of times it is brought on by a food allergy (useally fish or shellfish). It it very difficult to "cure" as the rash (sometimes small red bumbs, regular looking rash, hard scaley bumbs, small white bumbs, "white" rash,the list goes on on how it can look) can appear the next day or even over a week later.
Put cortizone cream on it to alleviate the itching.
It can disappear in a few days or may even take weeks (especcially if you keep eating or being in contact with what you're allergic to.
Go ahead and tell your mom and get a dr. appt. Assure her you are not sexually active and you think you're allergic to something (since it's on your chest too, that's useally not an std.) Has she changed laundry detergents or fabric softener? That would explain why you got in on your crotch area first (panties are up close and tight, Shirts aren't).
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