One day of heavy bleeding?

A good friend of mine had a miscarriage in May. She had a normal period in June. Keep in mind her cycle is like clockwork. She ovulates on the day her calendar says and starts her period when the calendar says.  Then this month she starts her period 2 days early has one day of heavy bleeding and then it just stops. It was such on odd thing to happen that she took a digital pregnancy test and it was negative. Is this normal? What could it mean?