I thought I would share that the month I fell pregnant with Annabelle I had spotting around ovulation, this lasted 3 days so it is possible that it was an ovulation bleed you experienced.
They do say that the earlier you have the scan the more accurate it is so I would proberly go with the dates that they have given you at this stage. Do you have another US planned? I would think that having one at 12 weeks would confirm more accurately how far along you are.