RANCHO MIRAGE, Calif. (CBS NEWS) -- Lexi Thompson had no idea why LPGA Tour rules official Sue Witters was approaching her on the way to the 13th tee at the ANA Inspiration.

When she found out, Thompson still couldn’t believe it.

A television viewer’s email had alerted officials to a day-old rules violation by Thompson for a 1-inch ball placement error . Her three-shot lead had just been wiped out by a four-shot penalty.

“Is this a joke?” Thompson asked Witters.

After being assured it wasn’t, she responded: “This is ridiculous.”

Thompson survived the shock and tears, and she forced a playoff with three gutsy birdies that had the Dinah Shore Course crowd on its feet.

But So Yeon Ryu managed to take advantage of the break created by Thompson’s extraordinary penalty.

Ryu birdied the playoff hole to win the LPGA Tour’s first major of the year Sunday after Thompson was blindsided for an infraction she had accidentally committed 24 hours earlier.

Thompson, the 22-year-old U.S. Olympian who won here in 2014, was left stunned by the decision that stopped her from cruising to what looked like an easy victory. The ruling cost her more than a strong chance at her second major title: Ryu won $405,000 with the victory, and Thompson went home with just over $250,000 in second place.

“Every day is a learning process,” said Thompson, who still stopped to sign dozens of autographs after her heartbreak. “I wasn’t expecting what happened today, but ... it happens, and I’ll learn from it and hopefully do better.”

The fateful email arrived during Sunday’s final round, alerting officials to the violation committed Saturday. Thompson marked a 1-foot putt with a coin on the 17th green during her third round, but she replaced the ball perhaps 1 inch out of position.

After an extensive video review , Thompson was penalized two strokes for an incorrect ball placement and two strokes for an incorrect scorecard. Witters regretfully explained the penalty to Thompson.

“I can’t go to bed tonight knowing I let a rule slide,” Witters said. “It’s a hard thing to do, and it made me sick, to be honest with you.”

Thompson fought back tears after getting the news, but she incredibly birdied the 13th hole. She battled back into a five-way tie for the lead, making three birdies and a bogey on the final six holes of regulation at Mission Hills Country Club.

“It’s unfortunate what happened,” Thompson said. “I did not mean that at all. I didn’t realize I did that. I felt strong through the finish, and it was great to see the fans behind me.”

Golf fans on the course and the internet reacted with bewilderment and outrage when the LPGA’s decision became understood. Tiger Woods immediately came to Thompson’s defense on Twitter.