I got the new baking powder (Magic baking powder by Kraft foods)and cake flour (Robin hood cake & pastry flour) at the same time and ever since all my cakes has a pretty strong chemical after taste and it is the strongest in the peanut butter cake I made most recently. Any idea which one is the culprit? Or is it my taste bud that didn't like the taste of cake flour? did it happen to anyone else?

My guess would be the baking powder. I don't know what could be in cake flour that would make it taste like chemicals, unless it's the bleached kind. I use Rumford brand baking powder and Soft As Silk cake flour and I've never had any issues with those.