My gum is inflamed, what causes this to happen?

Plaque: Plaque is the mean reason for gum inflammation. Lack of proper and adequate oral hygiene is what allows plaque build up. Some less likely factors for gingival inflammation are medications, viral infections, hormonal changes, or some rare gingival conditions or autoimmune diseases.

Probably Infection: There are several common causes of gum inflammation. However, by far, the most common is gingivitis or periodontitis (the advanced form). Both are caused primarily by oral bacteria which form into sticky plaque which clings to your teeth. If this is not removed every day, and regularly by a professional, it leads to swollen, red, puffy gums which bleed easily. See your dentist and get some help.