First of all, you do not need to take additional folic acid if you are taking prenatals. The prenatal should have an adequate amount of folic acid. If this is your first month taking prenatals, it could be causing the nausea and the headache. For the first month I was on prenatals I cut the pills in half. Now I take the whole tablet, but I take it at bedtime to eliminate these problems.
The only side effect that I know of for Vitex is that some women complain that their sides (ovary area) hurt or that they are extremely cranky. I'm not sure about the backache. Is this near your O time or period? Could be the culprit.
Good luck!! Hope you feel better soon.
Vitamin backfire? 
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