Well, it could be due to a combination of factors. You might have had an irregular eating schedule, lots of sugary drinks, and spent too much time sitting instead of being active. Stress and lack of sleep can also play a role in weight gain during high school.
The time when Jia Ling started to gain weight was not explicitly mentioned. The search results mentioned Jia Ling's slim image in the past and her fat image now, as well as the story of her and Qu Ying eating all kinds of Hunan delicacies together, causing her to become fat. However, there was no specific mention of when Jia Ling started to gain weight. Therefore, it was impossible to accurately answer the question of when Jia Ling started to gain weight.
Some of the common ways girls get fat in stories are through unhealthy eating habits like snacking constantly on junk food. Also, they might have a sedentary lifestyle, spending a lot of time sitting and not being physically active. Sometimes, certain medications or medical conditions can also cause weight gain for the girls in the stories.