What Causes Irregular Menstrual Periods?

Over the last 2 months I have had irregular bleeding. My cycle is usually 27-29 days. It started when my period was due 27-29 June but it didn t arrive until 8 July. Then my period arrived again 3 weeks later and it lasted for about 5 days. Then I continued to have red/brown/black spotting for 2 weeks and now I have my period again. It is very heavy, red and lots of clotting. I am 28 years old, probably slightly overweight but not by much, exercise regularly, generally eat well and don t drink too much alcohol. Do I need to see my Doctor?