Ok I dont know, so I'm just going out on a limb here. But you mentioned an ammonia smell. My sweat smells like anonia very strongly and it is from not eating enough Carbs and results in burning protein which makes your sweat smell like ammonia in short. No idea if that could be the problem tho, just throwing it out there.

I tried to google this too But only got the answer of infection of vaginal tract, or uti. But I also read it could be sweat from down there. Or it could be that you bleach your underwear and it creates that smell, but I think you would know if it was just the underwear.