By Torah law a woman becomes a niddah only when there was a hargashah. Here are three cases where halachah assumes there was one, even if the woman noticed nothing.

A reminder: the siman on stains contains many details that affect one another. Until we finish it and have all the information in hand, do not draw conclusions about real situations. If a question comes up, even one that seems already covered, seek halachic guidance.

What we have learned — the six conditions of a stain that renders a woman temei'ah

  1. The stain is larger than a gris.
  2. The stain is on a surface that can receive tum'ah.
  3. The stain is on a white surface.
  4. The stain is on a place or garment that uterine blood can reach.
  5. The stain is a problematic color (many shades of brown are not).
  6. The stain cannot be attributed to a non-uterine source.

And we have learned that the laws of stains apply to stains, not to bleeding. Two parameters distinguish them:

  1. The presence or absence of a hargashah
  2. The quantity of blood

Three cases — a masked hargashah

We saw that where there was a halachically significant hargashah, any blood the woman sees counts as halachic bleeding rather than a stain. In three situations we assume the woman experienced a hargashah even if she did not notice it:

  1. When the stain is on a bedikah cloth that was inserted, or on any other object inserted into the vagina (a tampon, a diaphragm, an ultrasound probe).
  2. When the stain is found immediately after marital relations.
  3. When the stain is found immediately after urination. (This last case is subject to an Ashkenazi–Sephardi dispute.)

What the three have in common is the concern that other sensations masked the hargashah: it may well have occurred without the woman noticing.

A stain on a bedikah cloth or an inserted object

A woman who finds a red stain on a bedikah cloth, or on anything else that was inserted, is a niddah.

If she has reason to suspect the stain came from a wound, she should ask a halachic question.

A stain immediately after relations

A red stain found close to relations renders the woman a niddah. The practical question is how long "immediately" lasts.

So a woman who finds a stain close to relations should ask.

If the husband found blood

If her husband found blood on himself after relations, she is a niddah even if it was not immediate, because such a stain has the same status as one on a bedikah cloth. If there is reason to suspect the blood is from a wound, ask.

A stain immediately after urination

A Sephardi woman

The stain counts as an ordinary stain and receives the ordinary leniencies. If the stain is on toilet paper, for example, she is not a niddah according to most poskim, since toilet paper cannot receive tum'ah.

An Ashkenazi woman

If she wiped immediately after urinating, she is a niddah. If she waited before wiping, she is not. How long she must wait is a machloket:

Practical halachah

An Ashkenazi woman who urinated, wiped without waiting at all, and found a bloodstain is a niddah. If there is reason to suspect the blood is from a wound, ask. If she waited some time but is unsure how long, ask.

Expanding the sources

The Gemara in Masechet Niddah (57b) describes three situations in which we are concerned that a woman experienced a hargashah without noticing it: during an internal bedikah, during relations, and during urination.

Regarding a stain found on a bedikah

The Beit Yosef discusses the Rishonim who hold that blood of any size on a bedikah cloth renders a woman temei'ah. He does not invoke the Gemara's reasoning that the bedikah itself masks the hargashah. Instead he explains that an internal bedikah is so close to the uterus that the blood almost certainly came from there, unlike a stain found on a dress.

The Shulchan Aruch rules that blood of any size on an internal bedikah renders a woman temei'ah.

According to the Kreti U'Pleti, Sidrei Taharah, Chavot Da'at, Shulchan Aruch HaRav and Taharat HaBayit, she is a niddah of Torah-level doubt. According to the Aruch HaShulchan and Darchei Taharah, she is a niddah by Torah law outright.

At first glance the difference between a Torah-level niddah and one of Torah-level doubt looks academic. In ordinary circumstances there is no practical difference: she is forbidden to her husband. In extreme circumstances the distinction is decisive. The Rambam writes that when a matter is in doubt, the doubt itself renders the law rabbinic — and where the law is rabbinic, poskim have room to find leniencies.

Regarding a stain found immediately after relations

The Mishnah in Masechet Niddah (14a) describes the "shiur," the span of time that counts as "immediately," and records a machloket:

Both measures describe a relatively short span, but the Ra'avad writes that we are not expert in these measures and must therefore be stringent. "Immediately" means any time close to relations.

Other Rishonim hold that "immediately" is a far shorter span, close to the language of the Gemara.

The Shulchan Aruch rules that "immediately" is the time it takes to take a bedikah cloth from under the pillow and examine. The Rema rules like the Ra'avad: as long as the blood is found close to relations, the woman is a niddah.

Which raises the obvious question: how close is "close"?

Regarding a stain found immediately after urination

This is a siman in its own right, and the details belong there.