Rising Louisville senior Kelsi Worrell, second from left, was part of the victorious 4x100 medley relay team at the Pan Am Games in Toronto in July. (Erich Schlegel/Usa Today Sports)

A year ago, Kelsi Worrell had never won a national title or competed in an international event. The rising Louisville senior was a relative unknown whose career highlights included a runner-up finish at the women’s NCAA swimming championships and a third-place finish at last summer’s U.S. nationals in a non-Olympic event.

What a difference a year makes.

Five months after winning her first collegiate national title and a month after Worrell took home her first international medal, a gold at the Pan American Games in Toronto, the 21-year-old claimed another first Thursday night at USA Swimming’s national championships in San Antonio. Worrell captured the national title in the women’s 100-meter butterfly. Her time of 57.27 seconds was three-hundredths of a second shy of her Pan Am record that ranks her as the third-fastest swimmer in the world this year and the fastest American since 2013.

Worrell’s time would have claimed bronze at the world championships had she been a part of the U.S. delegation that has struggled this week in Kazan, Russia. The surprise, though, was she didn’t even expect to be here.

“I didn’t, no,” Worrell told reporters at Northside Swim Center when asked whether she thought 2015 would bring her such a torrent of success. “I’ve been healthy this year. I think that’s the biggest thing.”

Worrell stepped into the national limelight as the surprise story out of the women’s Division I championships in March. Worrell claimed the title in the 100-yard butterfly while breaking a 13-year-old NCAA and American record held by Olympian Natalie Coughlin. She followed that up with the Pan Am gold last month before traveling to Texas this week.

That experience could prove valuable next summer, with Worrell now among the front-runners for a podium spot at the U.S. Olympic trials, which take place just more than a month ahead of the Rio Games.

A year ago, the Americans looked weak in the butterfly events, but with the unexpected rise of Worrell and the return of Olympic champion Dana Vollmer earlier this year, what looked like an event in transition entering the season has begun to solidify a year before the U.S. Olympic trials.

Vollmer, 27, finished fourth Thursday night in just her second finals appearance of the summer and second meet since the 2013 world championships. Vollmer had a baby five months ago and has been in the water training for less than four months.

West Potomac sophomore Cassidy Bayer, swimming for Nation’s Capital Swim Club, finished fifth in the final in 59.00, a personal best for the 15-year-old. Bayer’s time qualifies her for the FINA World Junior Championships in Singapore on Aug. 25-30 and moves her into fourth all time in the 15-16 age group, edging Vollmer’s best at that age.

“I was really hoping for a 58, but I was close enough,” Bayer said via phone. “My finish was off, and I know what I have to do to get under [59 seconds], and luckily I get another shot at it.”

Mason Makos’s Amanda Kendall, a former All-Met Swimmer of the Year from Robinson, touched sixth overall in 59.12, and North Carolina junior Hellen Moffitt, a former All-Met out of West Potomac, finished third in the “B” final in 59.45.

Notes: In other local results, California’s Chuck Katis, a Langley grad, finished fourth in the men’s 100 breast “A” final; California recruit Andrew Seliskar, the reigning All-Met Swimmer of the Year out of Jefferson, finished a disappointing seventh in the men’s 400 IM; Georgetown Prep junior Matt Hirschberger finished second in the men’s 400 free “B” final in 3:52.27, fourth all time among 15-16-year-olds; Oakton senior Megan Byrnes placed fifth in the women’s 400 free “C” final; and Andrew Gemmell, son of Ledecky’s coach Bruce Gemmell, finished fourth in the “B” final of the men’s 400 free. Like Bayer, Seliskar, Hirschberger, Byrnes and Gemmell all represent Nation’s Capital Swim Club. Katis also swam for the club before moving to California.