Someone please help me?

So usually my period is 4/5 days long, the first day is always a very light brown. Then after that I get red blood for a day and half then goes back light brown til I’m finished. But this month it started as really dark brown for a day then bright red and hasn’t changed and I’m on day 5 it’s not stopping. My periods have always been the same so this is very different for me... does anyone know what it could be?