I would go to the chair with "I'm confused. I got my schedule...But my contract says..." Pause. And wait to see what the chair says about it.
If you want you can follow up with, "What is the pay for an overload of this size, anyway?"
You have a contract so are on relatively firm ground. However--do you need a letter of recommendation for these people? Do you hope to have the contract extended? And exactly how many hours are represented by "Y?"