Im having thin white milky discharge turns yellow when dries with itching outside vagina on left side with rash & salty odor could it be std?

Yes.: If a women has a thin white discharge that yellows upon drying and causes itching with a rash it very well could be an STD in a sexually active person. A quick visit to your physician should determine the cause and treatment.

Please bring this up with your Gynecologist who knows your medical history and can give you a through exam. If you don't have confidence in your Doctor see another for a second opinion.

It could be STD including trichomonas, or mycoplasma / ureaplasma/bacterial vaginosis which may be due to overgrowth of vaginal flora (overgrowth occurs in sexually active but not yet defined as STD) or strep which can colonize the vagina or candida ( fungal infection that commonly occurs in women) . F/U/ w/ MD urgent care or gyn for assessment and treatment as you need treatment.