One of the most common types of cancer for a teenager is bone cancer (sarcoma) or soft tissue sarcoma (muscles, joints, tendons, fat, or cartilage). There is no way that anyone here though can tell you what this is . . so you will need to see a doctor for a clinical examination. Cancer in teens is generally rare . . but you can not discount it . . there are about ten common types of cancer in teens and young adults . . and sarcoma that affects the bone or soft tissue is one.
Teens Living with Cancer: Ewings sarcoma
http://www.teenslivingwithcancer.org/cancerFacts/whatIs/ewingsSarcoma.asp
Teens: Osteosarcoma
http://www.teenslivingwithcancer.org/cancerFacts/whatIs/osteosarcoma.asp
Teens: Rhabdomyosarcoma
http://www.teenslivingwithcancer.org/cancerFacts/whatIs/rhabdomyosarcoma.asp
Go see a doctor so that hopefully you can rule out sarcoma.
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