Period during breastfeeding

Any mom experiences irregular period during breastfeeding? I had my first period 4 mth PP and since then till 6 mth it's on a regular 28 days cycle. But suddenly this month I got my period 10 days early! I thought it was implantation bleeding but it gets heavier like a normal period. I never had irregular periods before being pregnant. I'm not sure if it's the breastfeeding or the stress frm work cz I just returned to work...