If what the employer said was true, you would not have a cause of action. The law does not require an employer to disclose good or positive information about an employee. The only restriction is that the employer cannot state any untrue negative factual statements about you which damage your reputation, since to do so would be defamation. However, a true statement is not defamation. So if you did call off that time to work a different job, the employer may legally say that, and is not required to also say that you had worked additional shifts to cover that time.