Can a bacterial or yeast infection of the vagina cause estrogen levels to drop? or cause irregular periods?

No: Estrogen levels won't drop. However you can have irregular bleeding.

Estrogen Levels: No, kathy, but sometimes it's the other way around- fluctuating estrogen levels control the vagina's micro environment and may make it more or less prone to infections. However, vaginal infections can cause "spotting, " inter menstrual bleeding , and irregular periods.