What Causes Sudden Irregularity Of Periods On Being Sexually Active?

I had a baby back in July and my husband and I decided to have another one back to back. I started charting my menstrual cycles to calculate ovulation. December through February all have been normal 28 day cycles and I was estimated to begin my next period on 20 March. However, I got it 8 days earlier. Is this normal or is there a reason it could be a whole week off?