To really find out set up an appointment with a dermatologist or your regular doctor. It does not hurt but it has been nagging you so it would be a good idea to check it out with the doc.
It might be a cold sore, cold sores stay for two weeks or more (or less). They usually mean you have genital herpes, but 50% of the world has genital herpes by statistics and only bad outbreaks occur in small percentages of those infected.
It might be a pimple that is not able to heal properly because it is on your lip and you move your mouth quiet a lot.
It will probably go away on its own, or if you are very worried consult a doctor.
Good luck!
Allen :}