If your blood flow is heavy and leaves the uterus quickly blood clots are not usually created. But if you have a heavy flow and blood leaves the uterus slowly, blood clots develop. Blood clots are not uncommon during a period. They tend to be dark or bright red in color and come in irregular shapes and sizes. Small clots the size of raisins are usually nothing to worry about and can happen during a heavy blood flow. Larger and thicker blood clots, greater than the size of a quarter, are reasons to be concerned.