didn't you say you were only due for af over the weekend? that seems really early for a scan, you'd only just be 4 weeks wouldn't you? with DD2 they only just found a sac at my first scan and I was 4weeks+6 for it (didn't know how far I was, could have been further). I think repeated bloods a couple of days apart will give you much more definitive answers than a scan this early.
apparently the sac is only visible from about 5 weeks/when hcg reaches around 1100 so there's still hope.