A Scottish designer is “gobsmacked” after wore one of her dresses.

Swift, 34, wore another this week, when she stepped out in $2,450 and a green velvet Sweet Jane Olive mini dress from designer Jade Robertson’s fashion brand.

“[I was] just completely gobsmacked at first,” Robertson, 34, said in an interview with U.K. newspaper on Thursday.

“We had no idea she even had the dress. It turns out she’d actually placed the order for it herself. She’d actually ordered it and paid for it," she added.

The designer, who is based in Perth, Scotland, co-founded the online fashion brand with her husband, Stuart, and launched the store in 2015.

"Yesterday was probably the wildest day that we have ever had," she tells PEOPLE. "We didn't know anything about it so we didn't know that she even had the dress. It wasn't like a press gift or a PR situation. She literally placed the order and bought it herself. But we didn't even know that 'cause she did it under a fake name."

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"So when we found out she was wearing it, it was a shock and just really, really exciting. The office was bonkers, everybody was super happy.

We didn't really know what it was gonna do, what the effect would be, we were mostly just really pleased to see something by our music-inspired brand on such a music icon."

Swift’s latest Reputation-coded look comes as fans speculate that the star will soon be releasing the “Taylor’s version” of her 2017 studio album. This is due to her appearing in outfits that and snake motifs which dominated her style when her .

On Instagram, Little Lies expressed their delight at seeing the "Blank Space" singer wearing one of their looks.

“Big moment for Little Lies waking up to see Taylor Swift has been photographed wearing our Sweet Jane Olive Mini Dress!,” a caption on a Thursday post on Little Lies' began.

The brand's post continued, “Major ‘pinch me’ moment - she looks absolutely unreal and we LOVE how she’s styled it! Sign up for pre-order over at Little-lies.com to get your hands on Taylor’s look 💚."

Alongside the caption were snaps of Swift wearing the dress as she stepped out for dinner with friend at Lucali Pizza restaurant in Brooklyn.

Revealing how she found out that Swift had worn one of her dresses, Robertson told

The Times that she was informed on WhatsApp by a member in her design team, who has a brother that is “a mega-Swiftie” and spotted the dress during “his 6am Twitter scroll.”

The team then found the specific order, and now believe that Swift herself, or a member of her team, made the purchase for the dress using a pseudonym.

Meanwhile, Robertson also revealed that the Sweet Jane Crushed Mini Dress had sold out by the time she looked on Thursday morning after it had been in low stock since its release in August.

Despite the increase in pre-orders for the dress, the boutique owner said the cost of the dress “will stay exactly the same," she told

The Times. "I do love her music, but what I really love is that she’s worn this in a very Stevie Nicks way."

"She is literally the most famous person in the world," she adds to PEOPLE on Friday. "For such a small independent brand, it's so incredible of her to not only shop and support but to actually then wear it. She knows what effect this is going to have better than anybody. So very very grateful for it."