The common causes are:
Drug reaction: some new medications that you are taking might cause this type of discolouration.
Contact dermatitis: It is probable that you have developed an allergy to something, you should check out whether you have started any new soap, ointment or detergent within this period. It could be from some new article of underwear. It could also be due to some food that you took very recently. Watch out for foods like shell fish, sea fish, meats, cheese, etc. Also any new types of processed foods. If you do find anything which could be the cause, then try to avoid it.
One likely cause in sexually active persons is allergy to latex, in condoms or diaphragms. You should use non-latex condoms which are available. Link.
It could also be due to lubricant or spermicide that is being used. Change the brands or avoid it.
If you do not have a history of recent unprotected sexual exposure, then it will be safe to assume that it is due to some allergic reaction. You should take an anti-allergic tablet, an antihistamine. You can take Tab. Alavert 10 mg once daily (OTC). Also, apply a soothing lotion like calamine lotion, also available online.
Other conditions it could be is Melanoma, but that usually has a slightly raised surface. Hence you should consult your doctor for this if it persists to have this checked up.
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