Imam al-Shafi'i (may Allah have mercy on him) said: "And it was clear from the revelation of the verse that it is not lawful for a divorced woman to conceal the menstruation that is in her womb, and this also includes the possibility of pregnancy along with menstruation."
And it is narrated from Sa'id bin Salim, from Ibn Jurayj, from Mujahid (may Allah have mercy on him) that he said, concerning the saying of Allah, the Mighty and Majestic: "And it is not lawful for them to conceal what Allah has created in their wombs" [Surah al-Baqarah: 228]: "It is not lawful for a divorced woman to say: I am pregnant, when she is not pregnant; nor: I am not pregnant, when she is pregnant; nor: I am menstruating, when she is not menstruating; nor: I am not menstruating, when she is menstruating."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / Q15413
It is narrated from Ibrahim (may Allah have mercy on him) that, concerning the saying of Allah Almighty: "And it is not lawful for them to conceal what Allah has created in their wombs" [Surah al-Baqarah: 228], he said: Most of what is meant by it is menstruation.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15413
It is narrated from Mujahid (may Allah have mercy on him) that (it means) she says: I am menstruating, when she is not menstruating; or she says: I am not menstruating, when she is menstruating; or she says: I am pregnant, when she is not pregnant; or she says: I am not pregnant, when she is pregnant. All of this comes from the woman's hatred of her husband or her love for him.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15415