My period has gone wonky?

Ever since my son was born, my period has come every 30 days, in the evening 90% of the time, almost like clockwork. This past month I was 5 days late. I was supposed to start on the 1st, I started on the 5th. I thought for sure I was pregnant, but I wasn't. Where I was late starting last month, should I have expected my period on the 1st or should I wait until the 5th to begin to worry? I've never had irregular periods since first starting my period so this is all new to me. Me and my husband are not actively ttc, but if it happens it happens & we would love to have another child. Anyone know what could cause my period to just suddenly start coming 4 days later??