You're experiencing die-off. This is normal. Start off drinking very small amounts of the kefir & work your way up according to your comfort level. If it causes you a yeast infection douche with 1/2 water, 1/2 peroxide. Do this as often as needed.
Kefir is one of the best things you can do to replenish the good bacteria. If good bacteria is moving in, the bad has to move out to make way for it. It'll need exit somewhere. This is why you would experience the flare up.