It depends on whether you are an exempt, salaried employee not subject to overtime or a non-exempt, hourly employee, subject to overtime. A full analysis of the Wage Order(s) governing your workplace, and the actual duties that you perform dictate whether or not you are classified correctly. Generally, if you are performing a position in which more than 50% of your workday is not spent making independent business decisions, then your salaried amount would be divided by 40 hours a week/8 hours a day to determine your regular rate of pay.
For any hours that you work in California answering emails/calls after hours that exceeds 8 hours in a workday or 40 hours in a workweek, then would be compensated at 1.5 times the regular rate.
Most employers require that overtime must be authorized. So, if you were responding to emails and calls over the weekend in response to others, you should report them immediately to your supervisor and request compensation. Your employer is likely to instruct you not to perform work of any kind without authorization in the future, and that you may be subject to disciplinary action if it occurs again. Your employer still must pay the overtime incurred, but the only way the employer can police overtime is to require pre-authorization.