What Causes Nausea Before Periods Start?

I get my period every 21 days and before I get it I feel nauseated and constipated as well as I'm always tired sometimes get bad headaches and I have terrible cramps before and during sometimes I even have to urinate 40 times in one day. Is there something I need to change diet/exercise or something I can do so I don't have to go through this in that short time period?