A new, mysterious character could be responsible. Maybe there's a new student or a hidden enemy in the school who has some ulterior motive against Harry and decides to target him during potions. This character could have used a very subtle form of magic that's hard to detect, like a charm that slowly affects Harry's strength as he works on the potion.
It could be Malfoy. He often has a vendetta against Harry and might have found a way to make him collapse during potions class, either by tampering with his ingredients or distracting him in a way that leads to a mistake in the potion - making process.
It's also possible that it was Snape, in a more complex scenario. Although he is the potions master, he might have unknowingly set up a situation where Harry collapses. For instance, if he assigned a particularly difficult potion that was too much for Harry's current abilities, and due to Snape's strict and unhelpful teaching style, Harry couldn't ask for proper guidance and eventually collapsed from the stress and difficulty of the task.
Look to Chapter 135 for the clearest answer. It begins with The sun blazed down mercilessly, but thankfully, the path to the mountain..., keeps building through Zhao Lang was also very humorous, and even the usually stoic Jiang Li..., and lands on Are you healed from your injuries?" Zhao Lang.
The best chapter pointer here is Chapter 171, "Jiang Yan: There's a ghost!!!". It opens with The afternoon rain stopped and started again, making the air even more... and closes on They weren't any big shots; did they deserve her to personally run over to....
The best chapter pointer here is Chapter 128, "Which family created the white lotus persona to go transplant rice seedlings in the field?". It opens with Still hungover, Huo Ziyang had just awakened from a blurry dream when his...
If you want the clearest chapter reference, go to Chapter 100. Early on, The barrage was about to die of laughter.; later, the chapter leans on [Hahahaha, help, why is the young master always being chased!] [Next door,....
If you want the clearest chapter reference, go to Chapter 132. Early on, The new girl in the live-streaming room went crazy.; later, the chapter leans on Who is this handsome guy???" "Ahhh, so handsome, I'm gone, where is the....
If you want the clearest chapter reference, go to Chapter 148. Early on, Exiting the cottage, Director Sun picked up the phone.; later, the chapter leans on Before he could even greet the caller, Su Yinwan's anxious voice rushed....
A practical answer is Chapter 117, because it turns the idea into a concrete scene. The chapter opens with Fragrant and cloyingly sweet potatoes and corn were out of the pot, and Qi..., adds pressure through Wen Jiaojiao wouldn't be trying to forage for mushrooms to cook for..., and ends on Since.
Look to Chapter 134 for the clearest answer. It begins with After a brief noon rest, the production team was up to their antics again., keeps building through Just as several guests had finished their relaxing hot showers and were..., and lands on [Are you kidding me, is Jiang Li putting on an act.
The best chapter pointer here is Chapter 137, "Lai Fu's Death Has Another Cause". It opens with Jiang Li personally went to Lai Fu's tomb and performed a small ritual. and closes on At that moment, Qi Tianyu had just finished his farming tasks, feeling....
Chapter 117 is the strongest chapter-level answer because it starts from Fragrant and cloyingly sweet potatoes and corn were out of the pot, and Qi... and follows through on Since entering puberty, his mother had exerted strict control over his....