It sounds like it may be a cold sore (also called a fever blister).
Cold sores are caused by the herpes virus (a different strain of virus than the one that causes genital herpes). Cold sores are quite contagious and often passed on from one person to another just by a simple, innocent kiss.
As a result they are extremely common and children often get the virus in childhood from a kiss by a parent or grandparent.
They usually last 7 - 10 days before the virus becomes dormant again and the sore resolves. Cold sores are usually painful, although this varies from person to person.
They begin very tiny and get significantly larger as the days progress. The color is often a dark red, purple, even a dark purple. They are usually fairly round and smooth but can be somewhat more irregular in shape, especially when large.
If this sore does not go away within 1 1/2 weeks from the time it first appeared (and sometimes people can feel it developing before they actually see it), you should see your doctor or a dermatologist to verify the diagnosis and recommend treatment.
If it is a cold sore, there isn't too much that can be done unless the sore keeps reoccuring in which case antiviral medication is sometimes used.
Can you see it when your mouth is closed or is it on the underside (the inside) of your lip?
Was it ever white or did it start out as purple?
How long have you had it?
I can really only think of two possibilities.
You need to go the doctor and have it looked at. It is not sounding like anything that is common, but it may be an usual presentation for something that is common.
A purple spot on the inside of the lip sounds like a healing ulcer or sore. My guess would be that you bit the inside of your lip without realizing it - perhaps even in your sleep which is not uncommon. By the time you wake up, it has already started to heal.
You have to remember that being inside the lip the color of a bruise will look darker. As it heals and the blood is removed from within a purple color often results as the hemoglobin (iron containing part of red blood cells) is left behind.
It is also possible - although let me stress unlikely - that it is a cancerous lesion, although it would be most likely benign.
In any case, you need to have it checked out because it is odd for it to disappear for a period of time and then reappear. The only things that traditionally do that are viruses. Any while viruses can cause sore inside the mouth, I have never heard of them being purple. They are almost always white or light yellow.
So you have me pretty stumped! Please do me a favor and make an appointment with your doctor. After you see him/her please message me her (Private Message (PM) to MNKMDMA) and tell me what it is.
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.
Sorry I couldn't be more definitive.