What Can Cause Delayed Periods With Light Bleeding?

Hi My LMP was 16th jan 2011. I usually have a cycle of 28-30 days. And I dont take any contraceptive pills. This time i did not start with my period on normal time. But on 25th Feb 2011, I did see some dark brown color spotting. On 26th Morning and evening, there was little bleeding. And since then not much. My periods usually a pretty heavy. But this time its not at all the same. What could be the issue?