I would echo this. Talk to your manager and find out how bonuses are calculated and distributed. I would also voice your concern.

Yup. Silence gives consent. Don't be a quiet slave.

Indeed you should be polite and direct in discussing this surprise with your manager. However, it's a common (and bad) sign of poor management when a company doesn't communicate proper expectations with it's employees. In your field, and your city, and given the rest of the details you've explained, it does sound like an explanation is in order.