Most commonly, bleeding gums are a sign of gingivitis, a mild form of gum disease. Gingivitis is caused by a build-up of plaque along your gum line which can make your gums irritated, red and swollen. If untreated, this plaque build-up could progress to a hard calculus deposit, which is particularly bad for your gums, and because your gums are so aggravated, brushing results in bleeding. Remember to brush and floss daily to minimise plaque build-up and your risk of gum diseases.