"Six months after my youngest was born we ran out of money and had to go out on the road again and I was 88kgs and did not feel like a pop star at all.
"I started taking this drug called adderall which is like speed to lose the weight and then I got addicted to this drug 'cause it made me feel invincible and I could work long hours.
"Then I ended up on tour in America supporting Miley Cyrus and it was a highly sexualised tour and I'd spent the last three years pushing babies out.
"I was supporting this girl who was much younger and more attractive than I felt and I just started acting out in all manner of ways," she continued.
Allen has previously admitted to cheating on her husband Sam Cooper on that tour, which included female escorts.
Realising she had a problem was when she considered taking heroin, as she tells the podcast of her decision to go to rehab.
Allen married Stranger Things actor David Harbour in Las Vegas last year.
She said: "I started cheating on my husband and I had always drunk alcohol to take the edge off the drugs.
"Then I realised I was going into the mini bar and drinking the mini bottles of vodka and without the drugs.
"I was in LA and thinking, 'None of this acting out is working any more. Maybe I should try heroin.'
"But because I had seen what happens to people who have taken heroin I knew it was time to confront whatever it was and my demons."
