I have gone through your problem. You said some black stains appeared on your lips after you took some medicine for fever
. Black melanin
spots on the lips may be seen in a condition called as Peutz-jeghers syndrome
. In this syndrome in addition to the black melanin spots on the lips, there might be polyps in the small and large intestines. Usually this condition is seen in adults. But the risk of malignancy (cancer) is rare in this condition. When you had fever, taking the medicine and appearing of these black spots may have occurred at the same time. Of course these black spots may be due to other reasons also.
My advice is to consult a general surgeon
or GI surgeon first to rule out Peutz-Jeghers syndrome. If those spots are not due to this syndrome then you can consult a Plastic Surgeon. Sometimes black stains due to drug reactions usually fade off with time. So don't worry about this.
Good luck to you.