It could have been a "bruise" which can lead to heavy bleeding. It happens in 1 in 100 pregnancies. Its where the sac didn't attach correctly and moved a bit. You can still have a very happy healthy pregnancy even with it. Its also not uncommon to not see a fetal pol at 5 weeks.
How are things going? Do you have an update? I have a similar situation that I just posted about.
If it's not heavy bleeding and you're not passing clots it's probably not a miscarriage.