What Causes Nausea,vomiting And Stomach Pain After Periods?

Hi,
I'm a 20 year old female, and I've been having nausea, vomiting (I gag all the time) diarrhea, stomach pains usually concentrated on the left side and occasionally a sharp pain on the right side. These symptoms have been going on forever, I thought they would go away on their own like they did last time (I had this happen to me in the summer time too but it went away for a while). I also get severe mood swings and headaches that seem to last for days. I just had my period about a week ago, so I don't think I'm pregnant, can someone please help me?