What Causes Irregularities In Menstruation?

Hi doctor, I have been having irregular period for the past two months, these are the dates my period came April 7, May 4, May 26, June 16, June 30. So I was prescribed primolut nor but since two days I have started taking it and I m so bloated, bad headaches and nausea. Can you help me on that? I have a 21months old baby, and want another baby. Thank you