Before the Grammy winner realized she was expecting last fall, she thought she was on her period because she was spotting.
In October, Trainor was promoting her new album Takin' It Back on Jimmy Fallon, where she performed her hit "Made You Look."
"That day, I literally did a TikTok that was like, 'When you get your period on Fallon...'" Trainor, 29, tells PEOPLE. "And that was me implanting. I was making motherf---ing life. I was making life during my sound check at Fallon."
According to the Mayo Clinic, implantation bleeding is common and "thought to happen when the fertilized egg attaches to the lining of the uterus"; it "usually occurs around the time you would expect to have a menstrual period" and "typically occurs about 10 to 14 days after conception."
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Adds Trainor: "I didn't know [I was pregnant] because I bled in the beginning of this pregnancy to the point where I thought there were periods. So when I found out how far along I was like, 'What? No, I just got pregnant and how am I that far?' So I didn't know bleeding is normal."
The pop star is already mom to son Riley, who turns 2 next week, with her husband, Spy Kids actor Daryl Sabara, 30. And she chronicled her first pregnancy experience in her first book, Dear Future Mama, which will hit shelves on April 25.
After learning about her implantation bleeding, "I was like, 'I should have put that in the book!'" Trainor says. "Every pregnancy is so different and so wild."
Courtesy of Meghan Trainor
The book is billed as "A TMI Guide to Pregnancy, Birth, and New Motherhood from Your Bestie" — something Trainor wishes she had when she got ready to welcome Riley.
"When I was pregnant, it was in the middle of Covid. I had zero friends who were pregnant. I was alone, so scared," she says. "It's the book I wish I had when I was pregnant so I wouldn't feel so alone or so weird or so: 'What's wrong with my body? Is this normal? How am I supposed to feel?'"
Adds Trainor: "I'm an open book, and I love telling everybody everything about my grossness and all my truths. When someone goes, 'Oh my God, me too,' then it makes me feel like I'm not alone, and I'm like, 'I'm not weird, my body's not different. It's normal.'"
Courtesy HarperCollins 'Dear Future Mama' by Meghan Trainor
While Trainor shares her own experiences — from gestational diabetes to blood clots and blistering nipples — she also tapped some experts to contribute to the book as well.
"I know not everyone has access to the amazing people that I have access to. So my dietician is in the book and gives advice for every trimester. My trainer is in the book, and there's pictures showing you what you can do for workouts because you can still work out when you're pregnant, and it's safe — I didn't know that. And my OB-GYN is also in the book all over the place giving advice."
For more on Trainor, pick up the latest issue of PEOPLE, on newsstands Friday, or subscribe here.