Hmmm interesting. Good question. Also interesting that your condition then dissipates when your period comes. I'm not sure what the ammonia smell is either but here are a couple thoughts.
With a generally "foul" odor see below.
Typically this condition is called "bacterial vaginosis" not serious, although somewhat embarrassing. It can be easily treated. See bacterial vaginitis and treatment. See below site. http://www.medicinenet.com/bacterial_vaginosis/article.htm Se your family physician for a diagnosis of BV and he or she will likely prescribe Rx Metoanidazole 500...
For more itching and whitish discharge this would more likely be just a yeast infection that we would normally treat with Rx Diflucan 150mg, 1 pill only.
See this site:
http://www.womenshealth.gov/publications/our-publications/fact-sheet/vaginal-yeast-infections.cfm#b
See your family practitioner for both of the above prescriptions.
In the meantime, try a betadine douche this help alleviate the soreness and itching as betadine has both a little antibacterial properties as well as antifungal properties.
Hope this helps.
Best regards
Maybe a type of yeast or bacteria infection? Have you been and gotten tested for ALL possible infections etc?