Unless you have an employment contract, an employer may schedule you for extra work if they need or want to do so to make customers happy, even if the reason they need or want to do this is that you'd missed work for a medical reason. If you are hourly, you must be paid for all additional work you do, including overtime as applicable (more than 40 hours/week); they cannot make you work for free if you are hourly. (If salaried exempt, you can be made to work extra without additional compensation.)