Kirsty,
I had the same thing at the beginning of this pregnancy at 4 weeks and lasted for three days and was was just when I would normally have been due for my period and my ob told me it is just my hormones getting into pregnancy mode. It can also be an implantation bleed as well. I have had a scan at 5 weeks before and saw just a sac as it is just so early. I have everything crossed for you!