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Skin lesions and rashes can occur due to a wide variety of medical and infective causes (other than sexually transmitted conditions). Trying to self diagnose will always result in too much of distress and anxiety, and also causes the person to undergo unnecessary tests.
There is no need to undergo any tests on your own.
You should consult your doctor or a dermatologist who can examine you and gain more information from your history. He can then decide on any tests if needed. This will help in proper diagnosis of the underlying condition and appropriate treatment can be initiated accordingly.