It's probably the theobromine in cocoa; it's technically toxic to humans but only in really large quantities, so perhaps your tolerance is far lower, Sloane! I mean I'm just taking a shot in the dark. The reasons that it's so toxic to cats, dogs, and other animals is because they have a much lower tolerance according for the theobromine to body mass ratio. So a tiny bit can kill them. But our ratio is so high that it's super difficult to be poisoned by it. However, maybe theobromine is the reason that eating too much chocolate causes discomfort and sickness? I don't know for sure but it'd make sense!