One suggestion would be to respectfully say: “If you are sick, we are going to call a doctor and schedule an appointment. He will give us a bill for that appointment. So are you really, really sick?” If she responds, yes, make that call. If not, tell her to get up (assuming that is possible). Be always positive in your conversation. Respect her feelings. On the other hand, it may also be how your mother responds to health problems. If you suspect that her pretending may be a disguise and someone could be wrong, urge her to talk with the doctor.