These blood strings are small thread like clots. We know that whenever we bleed from anywhere from the body, our body tries to stop the blood loss by producing clotting factor. This clotting factor make the blood thick which leads to clot formation. During periods, body also produces clotting factors to reduce blood loss. Uterus produces anti clotting factors for smooth flow of the blood. When clotting factor is in high quantity than anti clotting factors, clots are produced which can be string like or thick coin like clots.