If you aren’t pregnant but are experiencing watery period blood, Dr. Elmore says several reasons can cause it. These may include your actual menstrual cycle, certain medications, or dehydration. “Watery period blood also can result from normal changes, hormones, infection, or fluid in the reproductive organs,” she adds. However, low estrogen levels can sometimes cause watery period blood, according to Moy. “When estrogen levels are low, the lining will be thinner, and there may be more watery and scant menstrual flow,” she explains.