It may be a canker sore. Canker sores occur inside the lip, often as a single roundish area, usually white, and are also usually painful.
However, less frequently, canker sores can become larger, irregular in shape, darker in color, and instead of lasting 1 - 2 weeks can last for months. During this time they can get larger.
We do not know what causes them, but theories range from bacterial infections to injuries that become infected with certain bacteria to viral infections.
A canker sore would be my best guess, but you need to have it examined.
Without seeing it and feeling its margins, texture, etc. there is no way to be certain.
Also, even if it is a canker sore, larger ones sometimes need treatment so that they heal properly and do not leave scars inside your mouth.