Im skinny but i have white stretch marks on my hips, bum and thighs?

im 21 years old. and im only 50kilos and im 157cms. I used to be prob about 5 kgs bigger and ive recently tonned up by baby fat. anyway ive got all these white stretch marks on the side of my hips my bum and the inside of my thighs. i hate them so much and when i put on fake tan it makes them more noticeable. im confused since im not a big girl. do i not have enough elasticity to my skin?