Excessive yawning and tiredness. Months.

Hi my mother is suffering from a problem since two years. She yawn too much and she feels tired all the time , she is not even able to do household chores like cooking , cleaning as she lost all her interest in any activity. She feels sleepy all the time. This problem is not continuous but occurred Evey two months , for two months she becomes fully active and for next two she becomes fully tired. I can't see her in this condition. We had all tests done like BP , sugar, vitamins, brain MRI but everything came as normal . She is only taking blood pressure tablet. She said I don't have any tension as people say that she is taking tension of small small things but as per her she is not. Sometime she starts crying as well. In general whenever someone has any decease it's impact always increase over the time or it should decrease but here we are not taking any medicine as we don't know what is the reason but if there is some decease why it is showing its impact alternative of two months.