Kylie Jenner opens up about pregnancy changing her body 'at a super young age'

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Kylie Jenner is opening up like never before about why she started using lip fillers and the insecurities she faces when it comes to her appearance.

The 21-year-old "Keeping Up With the Kardashians" star covers Glamour UK's Autumn/Winter issue, which hits stands on Thursday, and divulges what led to her much-discussed pout.

“I was super insecure about my lips," she admits. "I really wanted [the] illusion of bigger lips, so I over-lined mine like crazy."

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Kylie says it wasn't long before her fans started to take notice of her lips, adding, "And then people started getting obsessed.”

With her life in the public eye, the beauty mogul has dealt with her fair share of critics, but says she eventually learned how to tune out the noise.

“So many negative things come at me all the time. Of course, it does affect me, but you develop a thick skin," she says. "Nothing has lasted so long that I haven’t gotten through it, so whenever a new bad story or rumor emerges, I know it will pass, too. Everything in life is transient.”

Since welcoming Stormi -- her daughter with her boyfriend, Travis Scott -- in February, Jenner has matured a lot. In the interview, she admits that "pregnancy completely changed my body at a super young age," before adding, "But I really don’t care."

In fact, 7-month-old Stormi is a big part of the reason why Jenner stopped using lip fillers earlier this year.

"During Kylie's pregnancy, she wasn't getting her usual fillers or putting on much makeup," a source told ET back in July. "Kylie really fell in love with the natural look, and wanted more of that, so she decided to stop putting filler in her lips."

The source continued, “Despite the fact that Kylie is a master of branding and has made an almost billion-dollar business off her lips and cosmetics, once Stormi was born, Kylie realized she was a mother first, and that is a big reason she has been sporting the more natural look."

Over the years, the makeup mogul has become much more business minded, and was recently named one of Forbes' youngest self-made billionaire thanks to Kylie Cosmetics.

"The business has given me a purpose. I love to work, as mom [Kris Jenner] has installed such a great work ethic in all of us. If I didn’t have that, I don’t know what I would do," she tells Glamour. "I get to create, it exercises my mind and I am always thinking of how to make the formulas or develop new products.”

Above all, Kylie Cosmetics gives the reality star something to pass down to baby Stormi when she's older.

“I work hard for Stormi and being able to pass Kylie Cosmetics onto her -- if she is interested -- would be such a cool thing to do,” she shares.

Watch Kylie Jenner Line Her Lips With Her Eyes Closed in a Makeup Tutorial