Bleeding of the gums is sometimes referred to as gingival bleeding, and it may occur during brushing or flossing. The soreness can be accompanied by swelling of the gum tissues. Most commonly, gingivitis is the result of plaque buildup on the teeth around the gum line. Gingivitis starts when food debris mixes with saliva and bacteria which, in turn, forms dental plaque that sticks to the surfaces of teeth. If dental plaque isn't removed by brushing with toothpaste and flossing, it can become mineralized and form tartar, or calculus. Tartar is very hard, and only a professional dental cleaning can remove it.