That was until she looked up at a giant TV screen in the Braehead complex and saw a video of her boyfriend asking her to marry him.
Romantic Tony Moore had recorded a surprise marriage proposal video and persuaded bosses at the shopping centre, near Glasgow, to show it on their massive 23-feet by 13-feet TV screen in the central atrium.
And after getting over the surprise of seeing Tony, 43, propose on film, the 26-year-old was quick to say: "Yes."
Tony, from Laurence Drive, Millenium Court, Glasgow told Diane he was going to see a mortgage advisor on Monday.
But instead, he slipped away to the shopping centre with the couple's 14-month-old daughter, Lilly to make the proposal video.
To grab Diane's attention, the toddler was seen first in the film with the Michael Bublé love song, Everything, playing as the backing track.
Then Tony came into view, went down on one knee, produced an engagement ring and asked Diane to marry him.
The proposal video was played yesterday afternoon at the shopping centre, in between the latest music videos, news bulletins and adverts normally shown on the screen.
Tony - who also has a 10-year-old stepson, Declan who was at Braehead to see the unusual proposal - said: "I was looking for a memorable way of proposing to Diane and since we come to Braehead regularly, I noticed that the giant screen had been erected.
"I had been thinking about proposing for months, but after I got the idea everything came together very quickly thanks to the people at Braehead who also organised the video shoot for me."
Tony added: "The video was brilliant and I couldn't wait for it to be shown on the big screen.
"I liked the idea of proposing that way because, as well as being very unusual, it was also quite personal.
"Diane knows I like to push the boat out and don't do things low-key."
Watch the proposal here -