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What is a Hargasha
Chazal learn from the pasuk דם יהיה זובה בבשרה that in order for a woman to become a Niddah on a Biblical level she needs to feel the onset of her flow. That feeling is called a hargasha. There are three major types of feelings (hargashot) that would indicate that her period began and would render her a Niddah.
Many explain that if a woman felt that her uterus opened that is a hargasha.
Others explain that if a woman felt some liquid flow from the uterus into the inner part of her vagina that is a hargasha.
Others explain that if a woman felt a certain shock sensation that caused her to shake that is a hargasha.
Even if she saw blood and didn't have a hargasha she is a Niddah rabbinically.
Nowadays most women do not feel any of these conditions. Nonetheless, the poskim assume that women who experience their period are a Niddah on a Biblical level.
The rules of ketamim do not apply to a bedika cloth even if she says that she didn't feel a hargasha.
Feelings that are Confused with a Hargasha
There are certain phenomenon that can cause a sensation similar to a hargasha. Therefore, if a woman was involved in any one of these activities and then saw blood that could have been emitted from her body it could be that she is a Niddah Biblically and that blood is not considered a ketem.
If a woman urinated: A ketem on a wipe or tissue found after urinating if found immediately it is considered niddah blood but if it is found after a normal amount of time that a woman would wait to wipe for all of the drops to leave her it is only a ketem. A ketem found on tissue paper isn't a ketem since it isn't mekabel tumah.
If a had tashmish
If a woman did a bedika: Blood found on a bedika cloth according to most poskim is a safek deoritta and it might have come with a hargasha. Some hold it is only a doubt on a rabbinic level. Either way the rules of ketem do not apply.
If a woman was involved in one of those activities and states that she is certain that she didn't have a hargasha according to some opinions she is credible. However, many poskim are strict and say even in such a case there is a concern that the hargasha was masked and the blood is judged as though it came out with a hargasha.
Any blood found on a bedika cloth even if she says that she didn't have a hargasha is treated as though it is possibly Biblical and the leniencies of ketem do no apply.
↑Pitchei Teshuva YD 183:1 citing the Nodeh Beyehuda YD 55 and Chavot Daat 190:1 who hold that a feeling of a liquid flow is a hargasha, while the Chatom Sofer 145, 153, 167, 171 argues. Taharat Habayit v. 1 p. 14 agrees with the Chatom Sofer that essentially this isn't a hargasha.
↑Aruch Hashulchan 183:61 writes that woman do still experience hargasha even if they testify otherwise. Igrot Moshe YD 4:17:12 writes that it is possible for woman to have a slight feeling of a hargasha and miss it. Either way he says that if a woman knows it is her flow she is a Niddah but if she finds blood and doesn't have any feeling of any hargasha it is only a ketem.
↑Taharat Habayit v. 1 p. 18 cites the Kereti 193:1 who presents two interpretations of Gemara Niddah 57b which differ on the point of whether the blood found with a bedika is a safek deoritta or derabbanan. He proves from the Rambam Isurei Biyah 5:5 and Trumat Hadeshen 47 that it is a safek deoritta. He also cites the Shulchan Aruch Harav Kuntres Acharon 183:2 Rabbi Akiva Eiger 62, and Aruch Hashulchan 183:55 agree that it is a safek deoritta