J.,
You did not mention whether the 4 month old is nursing. Sometimes the hormones involved in breastfeeding can create irregular periods, particularly if it is coupled with birth control pills. However, not regularly taking the pill can effect your period also. It would help to set your pills up on a weekly organizer, and that way if you miss a pill, you will know. Make a regular time to take it. I have to take a thyroid pill on an empty stomach. I usually wake up to use the restroom at about 5 or 6 a.m., then I go back to bed for an additional one or two hours. I take my thyroid med the first time I get up every single day. It helps to have a specific time to take the pill and be diligent.