I keep getting canker sores in my mouth, not in the same spot but on my gum line. they go away within a couple days. what is it ?

Exact cause ?: Exact cause of canker sores is unknown. Stress, diet,citrus or acidic fruits and vegetables,sharp tooth surface or dental appliance, sometimes impaired immune system or vitamin deficiency.

Diet?: Check what you are eating for any obvious foods that can cause canker sores and then stop eating/drinking them. Sour candy or citrus can sometimes be the cause. Viral infections can also cause what you are describing. Try gargling with salt water to improve.

Many causes/triggers: There is no single cause of canker sores but they are often triggered by particular foods like citrus, tomatoes & gluten. Some are caused by sodium lauryl sulfate, commonly found in toothpastes. My favorite treatment is topical licorice extract, but homeopathy may also be helpful. See http://www.huffingtonpost.com/2013/02/11/canker-sores-causes_n_2647071.html for a more thorough discussion.