Hello,, The amount of blood lost during a menstrual period ranges from 20 to 80 milliliters (mL). Blood loss of less than 20 mL may be considered a light period. Your saying there sudden decrease of blood flow. But cannot guess exact loss. Perhaps you wait this time , if you have blood loss less than 20 ml next time ,consult doctor. A shift or imbalance in hormone levels is the main cause of light periods.