shortened AF?

Cara
So I started my period on the 10th. My periods always last 5 days starting with the heaviest the first day, then medium then light the rest of the days. This time it was heavy the first day, lighter than normal the second then the 3rd and now today are pretty much nonexistent, basically just spotting. And it has been brown spotting. Has anyone else had this happen? My periods are always 5 days long with "regular" bleeding all 5 days. What would cause it to be like this this time?