Irregular cycles with Vitex?

I've been taking 500 mg. of Metformin in conjunction with 400 mg. of Vitex daily for the last two months and have had two periods since I started taking Vitex (after nearly a year of not having a period), but now I'm wondering if I should stop taking Vitex as I've read of a few instances where Vitex has contributed to irregular cycles in women that typically have regular cycles. Has anyone had this problem?