What causes menstrual bleeding too frequently ie every 18-21 days?

I have been having my period anywhere between every 17 to 27 days with it getting progressively shorter between periods for the past 14 months. I have had a thorough exam and even a sonogram and nothing has shown up. I had an iud put in 6 month ago but this was happening before that was put in. I don't have any cramps just some heavy bleeding the first few days then my period lasts 7-10 days with the last one lasting 14 days. What should I do and should I worry?