Every month for several days usually around when ovulation predictor tests positive my vagina gets very dry and itchy, what is causing this?

Not sure: My only guess is that the itchiness and dryness is somehow related to a brief abrupt drop in estrogen that occurs mid-cycle that might lead to these symptoms, especially if you are prone to yeast infections. One way to preempt these symptoms would be to self treat with some over the counter yeast cream leading up to when you ovulate to see what happens!

Progesterone : Progesterone changes the vaginal cells among other things. Yeast living in your vagina may be particularly sensitive to the change in hormonal milieu, and cause the mild recurtebt inflammatory symptoms.