Suggest Treatment For Irregular Periods After Child Birth

I am a 20 year old female. I first got my period when I was 15 and it started out regular but became more and more irregular over the years. I became pregnant May 2009, and since having my son in February 2010 I have had my period once; and only due to the fact I had been on the pill for one month then went back off it. Now the only time I get my period is if I am on the pill. What could cause this?