Parashas Metzora · Passuk 15:19
Chabad.org — English translation + Rashi
Torah Temima points to Niddah 57b.
English: “In her flesh” — until she senses [it] in her flesh.
Sensation (hargashah) required
Chazal require a sensed onset of the blood for niddah impurity; a stain alone without sensation does not establish it in the same way.