Parashas Metzora · Passuk 15:19
Chabad.org — English translation + Rashi
Torah Temima points to Niddah 41b.
English: “Blood shall be her flux in her flesh” — the Rabbis taught: “in her flesh” teaches that she becomes impure internally as externally.
Internal impurity of niddah blood
Chazal hold that niddah blood renders impure even while still inside (as opposed to the male zav, who requires exit).