What Causes Bleeding Between Periods, On And Off Nausea While Being On Birth Control?

19 year old girl on birth control. over the past month and a half iv had nausea cramping vomiting that goes away with food. usually happen if i get up for early shifts at work. pregnancy test negative. recently started bleeding irregularly. Period has remained constant. but bleeding in between period. Diet has not changed nor has there been any other change in routine.