Canker sores. Canker sores are the most common recurrent oral lesions. Individual ulcers typically last 1-2 weeks. The condition is idiopathic (cause is not known) recurrent aphthous ulcers is usually self-limiting and no treatment is required. If concerned, see your dentist after 10 days.
Unknown. Aphthous ulcers (cancers) are of unknown cause. Any acidic food stings when it contacts the spot but is not the cause. Usually no treatment is needed. You can suck on a clove to numb it. Silver nitrate on a stick makes them heal faster. Sometimes they are disabling & should be seen by an expert.
A virus. Canker sores are caused by a virus that remains dormant in your system, until stress, a scratch, or your lowered resistance often causes the canker sore to appear. Laser dentists have had great success treating (without directly contacting the sore with the laser) and with minimal discomfort. Often laser treatment cuts the duration of the canker sore in half!