What Causes Nausea And Light Periods While On Birth Control?

I had a baby dec 28th by csection and I bled pretty heavily for 3-3.5 weeks. I started birthcontrol at about 6-7 weeks postpardom. Into the 2nd week of my pack, i started what I think is my period. But its very very light. I also have been very nauseated since starting. I took a pregnancy test last week, it came back negative. Is this normal to be experiencing this nausea, and such light periods?