Metallic Smell

Over the past year I've noticed a metallic smell down there and I can't really remember wether it was there before or not. It's very strong during my period and then I guess the intensity changes depending on where I am at in my cycle. A few months ago I had tests for bacterial infections and such and recently I had an annual pap. Apart from some abnormal cells both of those turned up negative for any kind of bacterial infection or std. Does anyone else have this problem? What could it be?