Your's is a common complaint, almost 80% ladies have. The pain may be mild to severe. the probable cause is release of ovum from the ovary which makes the blood vessels of the uterus
to initiate enlargement. The pain occurs around the mid day of your menstrual cycle. Consult your gynecologist to rule out other cause for this pain. Tell your profession before getting advise, because some drugs may produce sleep. Otherwise nothing to worry.