What Causes Delayed And Prolonged Periods?

I am 41 years old and have always had my period every 28 days. Three months ago I was 2 weeks late. I went to the doctor he checked me and ran some blood test. Everything came back fine. I missed my period last month. This month when I started I was very very heavy. I was very heavy for about 4 - 5 days it almost stopped but started back with a little more than a light flow plus cramping. It now has been 2 weeks and I am normally heavy (not near this heavy) for 2 -3 days then light for 2 -3 days.