What was your contract?

Refer to your contract – are you paid by the hour (ie, did the contract give you an hourly rate) or salary, or was your pay rate never disclosed to you?

At some point someone set your pay rate, and I doubt that they gave you the equations that come out this way – you should write them a certified letter stating your salary or hourly rate that you were given when you first accepted the offer, and the terms under which it was offered (hourly or salary) and then tell they your were underpaid. You worked such and such hours, and they paid you less than you were told you’d be paid.

How they account for hours and how they pay you is an internal affair – but they should be paying you against what they offered.

Don’t wait, take them to task – otherwise you’re signaling your willingness to let them walk all over you, or your inability to monitor your paycheck.

-Adam