Is there a law to protect me from an employer that's is allowing for my commissions and employment to be jeopardized by allowing others to workmyshift

I work as a commission sales in a retail store for tree plus years of success.it is recently the managers are going against company policy by allowing other sales to work extra days and hours. This creates less oppertunities for me to get and sell a customer. We are also responsible for hitting sales numbers. I work 40 scheduled hours and am now having trouble getting to my goals and the pay i was used to getting. I was told that should work extra hours. Now i may be faced with termination.