Period 5 days early??

I have always been regular, or at least within a day or two of when my chart said I would start. But just last week, I started my period 5 days early and it was much lighter than usual. Typically my flow is heavy for 7 days, but last week is was light and only lasted for 4 Days! Do any of you deal with this? Or have had a random abnormal period like that? Please help! Thanks!