Gingivitis is a sign that there is excess plaque in your mouth, which your body is fighting back against through inflammation. As your gums get inflamed, they can become puffy and bleed when you brush.
This happens because your mouth is full of bacteria. These bacteria, along with mucus and other particles, constantly form a sticky ‘plaque’ on your teeth. If you don’t brush and floss regularly, the plaque can harden and form ‘tartar’ (also known as ‘calculus’). Calculus increases the rate at which plaque forms, as it provides a base for the bacteria. This causes inflammation of your gums, or gingivitis.