This is probably not an answer, nor am I sure it is more of a coincidence than anything - but I was having a terrible time with monthy infections and irritations as well. I even had a regular Rx for a short time for cream that I just used on a regular basis when the irritation started. However, I went on the pill 2 years ago and the problems subsided quite a bit.
Things you probably already do (in addition to the other things discussed already) - drink lots of H20, cut back on the sugar, I buy the prep H wipes and use those on occasion - but don't overuse them to avoid additional irritation.
Be careful using tea tree oil - it can have a severe drying effect.