Niddah 63 - Fixed Period
The laws that we learned at the beginning of the Tractate refer to women who do not have a fixed period. However, for those women who do have a fixed period, some laws will be different. What is a fixed period? The typical fixed period is when a woman begins her discharge at the same time repeatedly. For example, if for three consecutive months the woman begins her period on the fifth day of the month, she has established a fixed period. Alternatively, if she begins her period twenty one day after the previous, she has established a pattern.
But we did learn about fixed periods also, at the beginning of the Tractate !? True, but here we are learning not only about time-based, but also about periods based on physical condition.
A different form of a fixed period is one accompanied by certain physical conditions, such as continuous stretching, yawning, or belching, sneezing, feeling pain in the area of her navel or in her lower abdomen, fever or shivers, heaviness of the head or of the body. If she feels any of these, and they are accompanied by a period, and this happens three times in a row - she has established a fixed period dependent on physical factors.
In all of these cases where a woman has a fixed period, her law actually has a leniency: if she was preparing ritually pure foods and saw blood, she does not have to suspect that this blood was actually there from before, but she is ritually impure only from now on. There is no retroactive twenty-four hour period of impurity.
Art: Mihaly Munkacsy - Yawning Apprentice