It could boost a hormone too far. Like if you are estrogen dominant and it boosts your estrogen even further it can just be too much for your body. Like CharlotteDD said: you don't need to be on birth control to have too much estrogen in your body already.
It could also be a side effect from one of the ingredients. For example fenugreek can drop your blood sugar levels. There are other herbs that can increase your blood pressure or act as blood thinners.
Look at the ingredients list and read up on the herbs listed in them or check the main ingredients in wikipedia or google. Ingredients are usually listed from highest content to lowest content.
It could also be a side effect from one of the ingredients. For example fenugreek can drop your blood sugar levels. There are other herbs that can increase your blood pressure or act as blood thinners.
Look at the ingredients list and read up on the herbs listed in them or check the main ingredients in wikipedia or google. Ingredients are usually listed from highest content to lowest content.