Sac but no baby ????

💙💗MAMA H💗💙 • Finally got a BFP at 11dpo due 15/5/15 trying to conceive baby#3! Pregnant !! 3rd baby !!
Hi guys - wondering if anyone can work this out better than me pls?? 
So... Here goes lol.
My last period started 20th January this year.i have a normal 28 day cycle every month. 
I found out I was pregnant on the 18th Feb. 
I Worked out dates going by first day of last period , this resulted my due date being 26th October.
This would make me 8 weeks this week.
I have been light/medium spotting for around a week blood being purple/brown and sometimes a light pink but no fresh blood.
I was sent to early pregnancy unit where they scanned me and there was a sac and yolk but no baby. 
Seeing as I was meant to be 8 weeks they said is unusual no baby BUT my sac was measuring only 11mm instead of the normal 25mm size at 8 weeks. The sac being that small indicates I would only be 5 weeks and 4 days.
Am worried now as I'm 100% sure of my period dates and now worried it's a failed pregnancy?!? Have another scan in 14 days to see if sac has grown and a baby has appeared. What's everyone's take on this???