Likely some sort of stimulant as you point out. It is hardly the first time either as there have been public cases like this numerous times over the last two decades. Some cases even ending with deportation. The one I remember most vividly was someone carrying an unlabeled bottle of ADHD medication that had been sent to them while they were in South Korea by their pharmacist mum in the US; that they then ran afoul of when entering Japan. Similarly, there was a case at the University of Tokyo in the 00s, where an overseas student got sent an (allegedly) unprompted package with cannabis (not a stimulant though) from friends abroad. Allegedly, they were expelled and we got university-wide, anti-drug campaigns with memorable slogans like: "Illicit drugs are illegal".

Due to their history, laws regarding stimulants are harsher in Japan than in many other places in the world [1] and this frequently takes people by surprise. Not that Japanese laws related to illegal drugs are lenient to begin with.

[1]: https://en.wikipedia.org/wiki/Illegal_drug_trade_in_Japan