Référence sur Le Lévitique 15:19

According to Torah law, a woman is not rendered teme'ah and is not forbidden to her husband until she feels a sensation that blood left her body, but the sages decreed that upon seeing a blood stain on her body or clothing that she is teme'ah and forbidden to her husband even though she did not feel the blood leave her body, and even if she examines herself [afterwards] and finds herself clean. She requires a hefsek tahara to examine herself and find she is clean, and afterwards to count seven clean days from the day of her examination, as though she had certainly seen [niddah blood] as will be explained later in section 196. And if she felt that her uterus opened to release blood but checked herself afterward and did not find anything, there are those who say she is still teme'ah.