8 weeks pregnant threatened miscarriage?

I started bleeding like a mild period, bright red blood and cramps,Thursday so i went to the hospital, they didnt see any tissue or clots but told me im having a threatened miscarriage.Its not ectopic, thats what they checked when they did the u/s. the gestational sac and yolk sac are in my uterus they just couldnt really see a fetalpole. Well Friday i completely stopped bleeding, and have to wait til Tuesday to find out anything.Is it normal to miscarry and only bleed for 1 day? And why cant they find a fetalpole at 8 weeks?