First, assuming you are an at-will employee, an employer can change the terms and conditions of your employment prospectively (or on a going forward basis) for any reason or no reason, provided it is not a prohibited reason as such as discrimination based upon race, religion, gender, sexual orientation, military service, disability or for opposing illegal activity. Giving less than 24 hours advances notice is permissible in the absence of statutory or contractual provision requiring more notice.
If you worked more than 8 hours per day, or more than 40 hours per week you are entitled to overtime pay. For the Division of Labor Standards Enforcement's excellent guide on overtime rights, please go to the link below:
If your wages were not correctly paid, you may have a claim for unpaid wages. You can file a free complaint with the Labor Commissioner online at:
Alternatively, an attorney can assist you and you may be entitled to recover your reasonable attorney's fees under Labor Code 218.5