late period lasting 1 day

My periods are always regular and usually last 7 days. last month it was a few days late and the bleeding was extremely light. it lasted 2 days then went away for 2 days and then i had 1 day heavy bleeding and that was it. this month i was 3 days late and only lasted for 1 day but the bleeding was heavy and dark in colour but now its gone. an anyone please tell me what could be causing this?