Please help explain this to me!!

So, I started AF on Thursday afternoon.. my periods are normally 6 days long and pretty heavy, but today is day 3 and its already tapering off to brown blood... & yesterday was probably a medium flow. I'm confused because my 3rd day is normally pretty heavy and full on red. If my period goes away completely should I take a pregnancy test?? I thought I was out. Is it normal for a period to become shorter all of a sudden??