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If you work less than 40 hours a week and are salaried, you aren't likely eligible for overtime.
Here is a link to the Ca. Dept. of Labor that details eligibility: http://www.dir.ca.gov/dlse/FAQ_overtime.htm
I'm afraid that you don't have a claim for overtime to begin with. If you're not working more than 40 hours a week, and the work is not authorized by your employer as paid time, I don't think your odds are very good.
Again, you can file a wage claim with the Dept. of Labor, but I don't think your chances of prevailing are very good.