Please help!!!

I had my baby 9 months ago and my periods are monthly but the week I'm suppose to start I don't I usually start a week later every month and then this last time I was over a month late! My husband and I are trying for another baby but can't seem to get pregnant, I'm on a 45 day cycle between periods and I don't think that's normal, could there be a problem? Every time I'm late I take a test but yet they are always negative!! Could there be something wrong with me?