Hello, Welcome to healthcare magic. From your description, the painful dark red patch on your lip could be a condition called as fixed drug eruption. It occurs due to intake of certain medicines, commonly antibiotics and pain killers. It can also occur due to preservatives and food colorants. Every time you take an offending agent, it flares up and can became painful and red/ brown. I suggest you to observe or to look out for these offending agents whenever you get a flare of the patch. Recurrent attacks can lead to the discoloration becoming permanent. To get rid of the existing mark, you can try lightening creams containing kojic acid. Q- switched lasers can also help. However, with either modality, the results are poor. Hope this answered your query. Take care.