The differentials for a description you gave would include oral thrush, leukoplakia, and ulcers. Oral thrush is a fungal infection, and the white spots are actually areas of infestation. Leukoplakias are likewise white areas where there is abnormal growth of abnormal cells, these are considered premalignant. The thrush may or may not be painful, depending if there are areas that are injured/bleeding.
Another possibility are ulcers from an immune-mediated disease such as lupus.
Best you see a physician as well to get a second opinion. There may be other features beyond the mouth that would give some important clues.