I am a 17 year old male. I have stretch marks under my breasts and on my upper arms/shoulders. why?

Question

No drastic weight change.

Answer 1

Stretch marks occur anywhere that body growth exceeds skin growth.   Stretch marks are common in puberty and in people suffering from metabolic syndrome,  which causes most obesity,  heart disease and diabetes II. 

Answer 2

You are bigger now than you were 5 years ago.

Stretch marks, also known as Striae or Striae distensae, are a form of scarring on the skin with an off-color hue. Over time they may diminish, but will not disappear completely. Stretch marks are caused by tearing of the dermis during periods of rapid growth of the body, such as during puberty or pregnancy.

Answer 3

are you weightlifting ? maybe you are gaining a lot of muscle really fast