Back to the dreaded 2ww

According to my lmp I should be 7 weeks exactly today. First ultrasound yesterday showed only a tiny empty sac measuring 5mm or about 4 1/2 weeks. Doc took blood and put me on progesterone and said he wouldn't have any answers for me until I have another scan in two weeks to check for progress. I have had brown spotting for the last 5 days but my pregnancy symptoms stay strong. So confused and concerned. Stuck in limbo for the next two weeks. I won't even have the results of the blood work until Monday. Is it possible my dates could be that off even though I got my bfp 2 1/2 weeks ago? Maybe a late bloomer? Anyone have similar stories and what was the outcome?