"I couldn't believe it until I saw my name on top of the scoreboard, and saw my face on the TV screen at the venue," Siobhan says.
The Form Five student at St Paul's Secondary School, in Happy Valley, won gold in the junior women's 100 metres freestyle event in a championship record time of 54.47 sec.
Lithuania's 16-year-old Ruta Meilutyte, who won a 100m breaststroke gold medal at the London Games, finished in second place.
Siobhan, who is trained by Michael Fasching, head coach at the Harry Wright International Swimming Club, has been in fine form this year.
However, she chose to compete only when she felt she was ready.
In April, she decided not to race at the prestigious senior World Championships, in Barcelona, Spain, despite qualifying for the long-course 100m freestyle event.
"I finished my school exams only at the end of June," she says.
"Since these championships took place from July 19 to August 4, I knew I'd have only a month to improve my form.
"I thought that there wasn't enough time to prepare for such an important race."
She also turned down the chance to swim for Hong Kong at August's Asian Youth Games, in Nanjing . "The Games clashed with my only break of the year," she says. "It's a tradition for my family to stop what we're doing and relax every August; it's the only time for me to get away from the hectic school life and swimming training."
However, when Martin Grabowski, the Hong Kong team's head coach, asked Siobhan if she would swim at the world junior championships, she agreed because it fitted in with her schedule. "I hadn't heard about this competition before," she says. "All I knew was it was the first time Hong Kong would send a team."
In her first race, she beat her personal best of 55.77 with a time of 55.02. In the semi-final, she recorded a time of 55.04.
Siobhan was excited about racing Olympian Ruta in the final. The Lithuanian holds the world records in both the 50m and 100m breaststroke events. She also won three gold medals at the Dubai world junior championships - in the two breaststroke events and the 50m freestyle.
However, Siobhan paced herself perfectly in the race and steadily moved up on Ruta and the other finalists. She showed her strength to go in front in the last 25 metres.
Siobhan says the secret of her success this year has been balancing her swimming with her academic work.
She says she is lucky that she does not have to waste lots of time travelling between her home, school, and the swimming pool where she trains. "I live in Happy Valley and my school is also in the district," she says. "I have regular training at the South China Athletic Association in Causeway Bay, which is also very close to my home.
"It means I can use my time after training to do homework and revision, instead of having to sit in the car stuck in the traffic."
Siobhan will swim for Hong Kong at next week's East Asian Games in Tianjin , but has her sights on next year's Asian Games and the Rio Olympics, in 2016.