What Causes Stretch Marks In Lower Back, Hips And Breasts?

hi...i am a 15 year old girl....i got so many stretch marks on mt breast...lower back....hips n on back on knees due to which i cant wear short clothes n summers....i am very depressd with it.....i am 5'4 tall...n little flabby in dese areas......kindly..suggest me sumthng...so that i can enjoy myself like othr girls.....