I Recently Started Taking Black Cohosh After My Doctor And

I recently started taking black cohosh after my doctor and myself decided to stop my hormone therapy. I had been taking Premarin (a very low dosage) since 1995 when I had my hysterectomy. I am now 65 years old and started having real bad hot flashes after I stopped taking the Premarin so I started taking the black cohosh. That was about a month ago and I have been having bad headaches for the last 1-2 weeks. I thought at first it was stress but now I wonder. Could it be the supplement or still the stress?