What Causes Irregular Periods After Childbirth?

iam 23 years old.my baby was born on september 2009 .after that i got my period on november and then on jan.since then i didnot got my period .i stopeed
breast feeddung 3 months before and i moved to united states 2 months before.my periods are irregular.i took many pregnancy test all came negitive and iam
not on birthcontrol pills.i tried to take appointment from doctor unfortunately i got after 1 month so please advice me on this