Almost all of Christine Merrill's swimsuits used to be from Victoria's Secret. Not anymore.

Recently, the 31-year-old, who works in ad sales in San Diego, was planning a getaway birthday trip to Belize and noticed a sudden flood of Facebook ads for brightly colored swimsuits from a brand called Andie. Curious, one day she clicked on one of the playful ads to check out Andie's website. But she didn't buy anything.

Then, for the next few days, Merrill said, the ads seemed to get more frequent. With her trip to Belize looming, she went back to Andie's website again, noticed the swimsuit brand offered free shipping and free returns, along with a tool to suggest the best-fitting pieces, and she ended up buying three suits. She kept two of them and sent one back, thanks to a "very easy" returns process, she said.

"In the last few years, all these different [swimsuit] brands have popped up," said Merrill, who runs her own travel-and-lifestyle blog on the side. "I am certainly susceptible to the targeting, if it works. ... I feel like there was a time when Victoria's Secret was the only option."

When Victoria's Secret, then the dominant brand in women's swimwear, left the swimsuit business in 2016, it opened the floodgates for start-ups to conquer that market, which brings in more than $4.5 billion in sales in the U.S.

"When Victoria's Secret exited, we were all [trying] to recoup that market share. But we came at it with style and also comfort," said Lively founder and CEO Michelle Grant.

Lively started as an online lingerie business in April 2016. It launched swim in May 2017, when Grant said she noticed customers were wearing Lively's bralettes, meant for day-to-day attire, on the beach. "Couple that with at that exact same time Victoria's Secret was exiting a half-a-billion-dollar business," she said.

Now, though Lively still sells underwear and bras at its core, roughly 25% of total sales are of swimsuits during the peak summer season, according to Grant. And she said the company is exploring opening a standalone Lively swim store — in Los Angeles or somewhere in Florida — as Lively builds trust with women in that category. "Swim is one of the hardest categories to get trust from people," Grant said. Wearing a brand's bathing suit, "you're basically naked in public."