Hello respected, According to your history I think you are having any pigmented lesion on back of thigh . because if it was bruise than it should go away spontaneously in few weeks. asymptomatic brown black pigmented lesions can be seen in erythrasma or in pityriasis versicolor, both are very common conditions and believe me there is nothing to worry about. try miconazole cream , two times a day and combination cream containing salisylic acid 3 % plus benzoic acid 6% once in afternoon. Another thing it could be is residual pigmentation after a type of drug reaction , called fixed drug eruption.but can t comment further without history and clinical look at lesion, you can send me a photograph of the skin lesion if possible so I can assure you. Thank you