....Did she put something in it?
You bet she did.
If it was her present, that is. But the text states it's something she
received from Sou. And there's a reason for it. Because Feb 14th is not just Valentine's Day, it's Sumire's birthday. Not to mention, in that world - if something like Valentines even exists - the choco would probably be given by guys instead. Because, well... think about it. How would Suzuki react, if some girl gave him a box like that? lol... he'd think she wants to drug him.
And now imagine Sou receiving Valentine gifts from the 2-E girls.
Getting diabetes would be the least of his concerns.
You thought the hug fest was intense? Think about it, 36 girls racking their brains about what they can gift him... and how to make it, because just buying something is a no-go, you wouldn't stand out enough. Yuri will have a massive advanrage since she is making sweets for a hobby. Can't beat the Insanetess with store-bought stuff. And then they'd have to go to Sou right in front of the whole class, handing it over when his three overprotective wives (guards) are near him. And they probably would insist to taste-test everything Sou receives because they need to make sure nothing weird has been mixed in.