Yes, that's yourself you just got a whiff of. You're not that sweet-smelling little kid anymore.
As your hormones rage and your body plays tricks on you, your sweat glands are getting in on the act. Glands you've always had suddenly get a lot more active, and they add new chemicals to the sweat that makes it smell more. The specialized apocrine glands in your armpits now release a fatty sort of sweat that's a favorite food of some bacteria. When those bacteria start to feast on the sweat you produce when you're active, hot or stressed, they release a bad odor.
That new hair you have in some of those same places provides more nice, warm and moist places for the bacteria to feast.
The best defense is to get rid of the sweat and bacteria before they have time to interact. That means regular showering or bathing, with a mild soap, maybe one with antibacterial ingredients. Use clean washcloths and towels. Wear clean, comfortable, loose and absorbent clothing.
You may want to use a deodorant or antiperspirant. Antiperspirants suppress the production of sweat by plugging the sweat glands. Many of them include aluminum salts, which some people think may be linked to health problems. Deodorants fight the bacteria rather than suppressing the sweat. They may also include scent. Read the directions --some products, for example, work better while you sleep.
For more information on adolescent skin care, see the links on the next page.