TMI period question

Katie
Okay so I started my period Friday, normal red blood but yesterday my second day I had nothing but dark dark brown blood ALL DAY some clots too. Anyone know why? I've never had this happen before. Today, My third day I have dark blood again but very light and only when I wipe. Anyone have had this happened? Tried googling but not much answers. 
Mind you my period normally lasts 4 days
Input please!