Hello,
Thanks for posting your query.
Considering that your lesion is smooth , small and not increasing or changing in size , shape and structure it's not likely to be anything serious like a melanoma( skin cancer ) or any neoplasm .
However without a physical examination and a biopsy ( if needed ) a doctor cannot confirm the diagnosis of such spot.
The sudden appearance of dark spots on your lips may occur due to many reasons. Some of them may be minor ones like:
Smoking: Nicotine in cigarettes stains lips causing them to turn dark and sometimes cause spots.
Lipstick: Using old expired lipsticks or ones of inferior quality is also known to cause dark spots on lips. Avoid using un-heard of brands and stick to quality products.
Dry lips: Keeping your lips well moisturized may help prevent dark spots as dry lips are known to darken due to lack of moisture.
Sun exposure: Lips also darken due to prolonged exposure to sunlight due to pigmentation
Hormonal imbalances: Abnormal fluctuations in hormonal levels can sometimes cause dark lips.
Dark spots of lips may also be indicative of a bigger problem, such as:
Oral melanotic macule: Also known as labial melanotic macule, this is a non-cancerous (benign) dark spot found on the lips or inside the mouth. This condition can be diagnosed with a biopsy which can tell you if the spot is after all only a benign oral melanotic macule, or if it is malignant.
Peutz-Jegher's syndrome: This condition is also known to cause dark spots on the lower lip and other areas.
Dark spots on lips occurring due to minor reasons can be cured in many ways, some of them being:
1.Applying lemon juice, which is natural bleach, should help lighten your lips.
2.Cutting down on tea and coffee may help as they are known to darken lips. Instead, opt for healthier drinks like fruit juices and milk shakes.
3.Avoid prolonged exposure to direct sunlight and when out in the sun; use a mild petroleum jelly to keep your lips from drying up.
4.Avoid using a lipstick for some time as the dark spots may have been a reaction the lipstick you use.
5.Licking your lips often is also not advised as saliva is known to dry lips.
6.Nicotine is known to cause lips to darken and should be avoided.

So as of now it doesn't seem worrisome but I would still advise you to consult your doctor for confirming the benign nature of the lesion to be on the safer side.
Meanwhile you can try the above for self resolution of the same .
Hope I have answered your query.
I'll be available for any follow up queries.
Regards

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