The singer, who just took home a Grammy for best new artist, tells CBS Sunday Morning that she used to hide behind bulky clothing.
"I – every day – wore sweatshirts and sweatpants to cover up my body because I was so insecure," Trainor, 22, says on Sunday's episode. "And it would be summer. And I would go on vacation and I'd be in Trinidad and Tobago, 90-degree weather, and I'd be wearing sweatshirts."
Her biggest concern wasn't her 'bass,' but her arms, Trainor says.
"I didn't want to show my arms," she says. "I didn't want to show anything, and I was just so insecure and uncomfortable, and I thought, like, 'All right, if I'm fully covered – I'm good. And that's not what I should have been feeling.' "
Trainor previously told PEOPLE that her hit song finally boosted her self-esteem.
"Confidence was always something I wanted in my life," she said. "But I was always very insecure growing up. When I wrote 'All About That Bass,' I wrote it wishing I was that confident. Then one day, they told me I would be the one singing it. It helped me to believe what I was singing."
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"I get messages all the time, 'I hated myself, I didn't want to go to school. I was so uncomfortable, and now I love myself. And I was in a really dark place until your song came out,'" Trainor tells CBS. "And I was like, 'Whoa, man, we've got to do more of these. We've got to do more of these songs."