Most probable and most common cause of periodontitis is deposition of tartar on teeth. You need to get scaling(cleaning) done from a dentist. If ,even after scaling, the problem continues, then you can think of other causes .

Get deep scaling and curettage done from a dentist,start brushing after each meal.

Periodontitis is inflammation or infection of structures surrounding teeth which includes gums and other tissues. It is mainly caused by bacterias deposited on tooth surface but is secondarily aggravated by hormonal changes, hereditary factors, diabetes etc. Some oral infections may seem like periodontitis but they don't lead to periodontitis. First and foremost treatment is to get a proper deep scaling done from a Periodontist (gums specialist) preferably. You may not have periodontitis per se... it may be just infection of gums or Gingivitis.

Gum disease can be called as periodontitis. It could be because of harmonal changes, heriditary, pregnancy, Diabetes,etc. Various treatments from as simple as scaling to advanced flap surgery maybe required after taking xrays and finalising the problem.