What to do about an employer that's trying to stiff you out of your bonus? It's my first ffp, so please be kind ;)→[More:]

I have worked for my current employer for 1 1/2 years, and when I was initially negotiating my salary my two bosses discussed the bonuses that I would be receiving throughout the year (after completing 1 year of employment). They did this because the salary I was requesting was higher than what they wanted to pay me. The bonuses did bring my salary close to what I wanted, and I was desperate, so I accepted.

I should note that I work for a small company - 10 employees - and it's privately owned.

I received my Christmas bonus as expected, and was waiting for the end of year bonus. The company year end is January 31st, and last year I saw people receiving bonuses around March (I do the books so I was cutting the checks).

I know for a fact that two people did receive this bonus this year end. Although, I haven't been given the receipts like last year, my boss cut the check when I was on lunch, and therefore away from my desk. My employers aren't aware of that I'm aware of the two bonuses. This makes me very pissed, but I don't know how to handle this. What are your thoughts.. should I say something?? If yes...what?

PS I really want to bring this up, but I don't want to offend :)I just don't know what to say without coming off as a bitch.
I should state that I work very hard, and do things on my own time, like the bank deposit or getting office supplies. It's not like I'm a slacker. I definately believe I deserve this, I just need some insight in how to approach it.

Thanks everyone!!