M.P.
For what it's worth. My periods were never consistent as to length when I was on the pill and even when I wasn't on anything. Like yours they lasted 4-7 days. Were heavy one month and light the next. Doctors said it was normal. It has to do with the way your body produces hormones. Many people have irregular periods. I suggest that your hormones have still not settled down since your pregnancy.
But, only your doctor can tell for sure that everything is OK,