But anyway, like Asian Boyz Luv Me mentioned your muscles are growing too quick for your body, resulting in the most bottom layer of skin(dermis) to stretch out sense it can't keep up with the pace. It happens the same way with getting to flabby quickly. It can also be genetics or your body is changing (puberty).
Best thing to do is apply Vitamin E or Cocoa Butter on your skin and on your stretch marks. This will reduce and help the elasticity of the skin. Though you cannot get rid of Stretch marks, they do fade though.
Fun Fact: Purple Stretch marks are considered to be the most recent stretching, while the white/skin tone marks are older and are healing.
Muscle hypertrophy occurring at a rate beyond the elasticity of your skin. It's very normal, especially for people who just started working out and go hard. I still have some stretch marks on my chest from when I went from flat to whatever it is I am now in only a few months.