Hi S.,
With irregular periods, I strongly recommend making an appointment with an endocrinologist. (An endocrinologist is doctor who specializes in the glands and hormones that regulate mood, growth and development, metabolism, tissue function, and sexual and reproductive functions. Source: Yourtotalhealth.com.) Women who have been pregant or have given birth are often found to have hormonal imbalances, commonly caused by low thyroid function, aka hypothyroidism. It is very easy to diagnose and treat, buy many women (and some men) go years without help because their symptoms are not dramatic, at first, and include some of the following:
Feeling tired, weak, or depressed
Dry skin, brittle nails and hair loss
Not being able to stand the cold/being cold all the time
Constipation
Memory problems or having trouble thinking clearly
Heavy or irregular menstrual periods
(Source: Webmd)
Many primary care doctors and even ob/gyn mds brush off these syptoms as typical new mom/tired mom symptoms. Left untreated, they can be very harmful to your body (not to mention your marriage/life) and also harmful to a fetus if you become pregnant again while the condition is undiagnosed.
While I do agree with many of the responses that you will most likely just need a little time, it is much better for you to have an endocrinologist do a little bloodwork now to spare yourself the possibility of overlooking this kind of medical problem. I went years without being diagnosed and I can't begin to describe the toll it took on my mind, my body, my memory - little by little, day by day. And it all started with irregular periods that were "fixed" by birth control pills...and a pcp who said I was too young to have anything wrong with me... and giving it more time to get pregnant... and clomid when that didn't help...and then a baby who "cause" of all my fatigue, etc., ...well, you get the idea. Periods are supposed to be regular - period. If they're not, there is a reason. Don't let anyone (md or other) tell you it's just one of those things in life. Thanks to a wonderful psychologist who recognized the medical warning signs behind my depression, and a great endocrinologist, I'm doing so much better - and so is my family of three kids and Dad!
Best wishes from our family to yours.