The Dance Moms star told ET that the weight loss occurred when she was put on prescription medication, but noticed while shooting her Lifetime show in Australia last summer that their version of the drug was different.
"I was taking some medication for thyroid this and that and I had to have the prescription refilled there," she recalled of her trip down under. "Well, it was the same medication but the pills are different. Our pills here are coated for your stomach and this and that, not there. No matter what I ate I was like sick everywhere."
She added, "So, I think that was really the kicker when everyone said, 'Oh you look so good, you look so good.' I'm thinking, 'Well, I didn't do anything different.'"
Miller also told People, "There's no diet, there's no exercise, there's no this, there's no that," she added.
While the loss of lbs. wasn't intentional, the TV star is embracing her smaller size after getting sick for a month straight.
"That month was just enough to kind of say, 'Hey, maybe I should really start to watch,'" she said.
Miller also credits staying in Los Angeles for helping her keep the weight off. "I've been so extremely busy i don't have time to eat that much. Here, driving is more difficult. At home, I could eat pancakes on my lap while I was driving a car in Pittsburgh. ...That's hours of my day that I'm not eating."
As for how much weight she's lost, Miller said that isn't her focus, but knows she was down 18 pounds when she saw the doctor.
"I haven't kept a record or thought about it or been on a scale," she told ET. "Let me say this I've always had to go to a plus sized store and now I don't."