What Causes Irregular Periods After A Miscarriage?

Hello Madam

I have diabetics problem from past 4 years.
I got married on May-3-2009
My DOB : 23-12-1986
My Wife Age is : 18yr Now

Her period are very regular She got LMP on May-28-2009 and not on Jun-28-2009 so we done Urine Pregnancy Test Positive and Gone to doctor they give Folinz (Folic Acid Tablet) after reaching 10 weeks we scan for Aug-8-2009 the growth is only 6 weeks and no fetal heartbeat found so come scan for next week, but the fetus passed out while he urinating on Aug-12-2009 Night so he got a D/C at Aug-13-2009 and from thereafter he got very irregular periods.
The Next periods are
Sep-15-2009
Oct-13-2009
Nov-13-2009
Dec-19-2009
Jan-16-2010
Feb-21-2010
Mar-31-2010
Till May-10-2010 he did not got her period so we gone for doctor & found Urine pregnancy test Positive and then scan on May-17-2010 found 5.2 weeks gestational sac and no yolk sac or fetal pole so scan again in 2 weeks but he is bleeding from May-26-2010 and the tissues passed again and miscarry. why this hap