Your sweat and sebum in the vulva can ferment urine so that the natural ammonia
mixes with the secretions and bacteria to produce this ammonia
-like scent. I have found success wearing only cotton knickers and went from pantyhose to thighhighs. Also, a very dilute white vinegar warm water wipe down of the vaginal lips, especially around the urethra (pee-hole) and under the clitoris helps, but I would caution you to do this gently. Scrubbing will irritate and make it worse.