She had stomach cramps and nausea when she ate and blood in her stool afterwards.
In her last two years of secondary school, Tiffany Chee, 17, had to go to the accident and emergency department 15 times and was hospitalised five times. She missed many lessons and her grades suffered. It did not help that her family was also facing financial problems.
"I was too scared to eat, so I stopped eating, and my weight got lower until I became underweight," said the former student of Junyuan Secondary in Tampines, the only child of a businessman and a clerk.
While doctors in the four hospitals she visited could not diagnose her problem, for Tiffany the cause seems clear. "I think it was really due to stress," she said, noting that the problems went away after she started exercising regularly.
Regular exercise led to better health for the Normal (Academic) student, who did well enough in the O levels to meet her conditional offer - made via the Early Admissions Exercise - to study biomedical science at Temasek Polytechnic.
She learnt yesterday that she had achieved a score of 16 for her L1R4 - English and four relevant subjects.
"Although I am not really satisfied, I know I did my best," said Tiffany, who was aiming for 14 points.
Tiffany, the first N(A) student to be student council president at her school, added: "Now that I've made it into my dream school, I will study even harder."
She is among the 30,292 secondary school students who sat the O-level exams last year. They outperformed their seniors, with 84.3 per cent attaining at least five passes, up from 83.8 per cent in 2015.
Tiffany, who now runs and hikes with her parents, said she learnt how to pace herself. Rather than overestimating how much she could achieve in a day, she started scheduling more breaks in between.
Tiffany, who wants to become a paediatrician, said of her challenging experience: "Even though it was the most difficult (chapter) of my life, it was also the most touching. I learnt that there were people who would be there for me."
Another student who did well is Maria Erika Glen Goh, 16, of Anglican High School (AHS), one of her school's top performers with a raw L1R5 score of eight points.
In her final year, the vice-president of the AHS Dance Society juggled leading roles in a school musical and ballet concert, on top of taking part in the Singapore Symphony Children's Choir, Singapore Repertory Theatre's The Young Company and Singapore Dance Theatre.
Said Erika, who hopes to attend a junior college but has not yet picked one: "I'd be very interested if (the JC) has a vibrant environment. I will look at how it treats the arts."
Meanwhile, St Hilda's Secondary School saw an overall improvement in its O-level grades this year, with a lowest raw L1R5 score of seven.
Said principal Khoo Tse Horng: "Our top scorers are people who already have shown they are excelling in all aspects. As they work hard in their CCAs (co-curricular activities), they learn self-discipline and time management, and this helps them build their focus."