Although it’s believed Charles and Camilla had cooled their affair right before the wedding, Diana had always been suspicious of their relationship, so spare a thought for her.
She even said she searched for Camilla, 70, in the crowd of 2500 guests as she made her way down the aisle of St Paul’s Cathedral on her wedding day.
“I knew she was in there, of course. I looked for her,” Diana told royal biographer Andrew Morton in 1991, as reported by Good Housekeeping.
“So walking down the aisle, I spotted Camilla, pale gray, veiled pillbox hat, saw it all, her son Tom standing on a chair. To this day, you know – vivid memory.”
So, why the hell was Camilla actually there in the first place?
Well, Charles and Camilla's relationship has been a long and complicated one.
Camilla's then-husband, Andrew Parker Bowles, was the Commanding Officer of the Household Cavalry Mounted Regiment, meaning he was the guy overseeing the ceremonial escort that led the newlyweds' carriage ride through London. So, he was basically working at the wedding.
Also, despite having dated in the early '70s, Charles and Camilla continued to be close friends even after they broke up.
If it wasn't for his parents disapproving of Camilla, Charles, 69, probably would have married her instead of Diana.
Camilla went on to marry Andrew in 1974, but she could never really let Charles go - even though they'd both moved on to other people.
Meanwhile, Charles downplayed the significance of his relationship with Camilla, initially telling Diana she was just an old friend. Haven't we all heard that one before...
"He came clean after the engagement, admitting that Camilla had been one of his most intimate friends, but reassured Diana that from now on there would be no other women," author Penny Juror wrote in her book, The Duchess: Camilla Parker Bowles and the Love Affair That Rocked the Crown.
Charles is also godfather to Camilla's son, Tom, 43, and although he thought Tom should act as a pageboy at his wedding, Diana apparently dug her heels in and said no way.
"She agreed to several of his other godchildren, but there was a big scene about Tom Parker Bowles and Charles backed down," wrote Juror.
While she was at the cathedral, Camilla missed out on an invite to the breakfast reception at Buckingham Palace following the wedding ceremony.
Still, Camilla was definitely still on Charles' mind, even at his wedding to Diana.
"Charles glanced over at Camilla as he walked down the aisle with a 'slightly plaintive, sad look' on his face," Juror said.
"Their wonderful affair was over and this was reality kicking in."
But it was never really over between the pair. Eventually, Charles admitted he reignited his love affair with Camilla in 1986.
He divorced Diana in 1996, and went on to marry Camilla in a small ceremony in 2005.