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