What Causes Burning Sensation Around Lips Followed By Blackening Of Mouth?

Dear Sir,
Few days back. i got a burning sensation around my lips.....and then after some days the skin around my mouth blackened...This has been happening quite frequently now......the Black marks go after some days..... But this time....i have developed a very bad rash in that area......My doctor has prescribed Flutibact cream and vozet tablets....Iam aprehensive to sue them.....Please guide.