Reference for Niddah 114:24

ת"ש

observed a discharge of blood, R. Meir ruled: If she was standing at the time she is unclean,<span class="x" onmousemove="('comment',' Since owing to the narrowness of the passage occasioned by her standing position, her urine may have returned to the interior of her womb whence it gathered up some menstrual blood. ');">²¹</span>

Jastrow

Jastrow