irregular periods

My period returned March 23 (8 months pp). The first 2 were pretty normal 28 day cycles but the last 3 have been odd. One was 7 days, next was only 4 and very light and this last one 8 days and pretty normal but long. I was suppose to get my period on Saturday but did not. I have had some spotting throughout my periods. Which I did this month but no period. We use withdrawl and have succesfully for almost 10 years. Our baby was planned. I took 2 pregnancy tests and both negative (fmu). My son was very sick for about a week last week and would not eat just nursed like a newborn all day long. Can this cause your period to be late??