Hello there, I am Dr.Milan an infectious disease specialist answering your question. MRSA is a very complicated organism to treat and to remove from body. But i think you know its MRSA than surely your doctor treated you very carefully and you recover. Now knot on labia major if it is because of MRSA than you have associated symptoms and fever should be there most of the time.Her it is not so i thing it is not MRSA. But it is one of the STD, Sexually trasmited disease. It is hard so it may be a viral like HSV and some time bacterial like Ducrey or syphilitic. But this both bacteria cause ulcer. One is painfull & other is painless. Local examination and history is indeed for the diagnosis of this type of infection. It can not be done by any other way simply. So my best advise you just visit a dermatologist and follow his instruction. Hope I have given appropriate guidance to you. if you have any query you can consult me anytime. Give me star rating, helpful vote & thank you according to your satisfaction level. Thanking you.