Dates WAY off...

Erin • Early MC Oct `14 & Missed MC April `15 Praying for my Rainbow some day
Just had my first u/s. 
I thought I was 8w2d today but tech only saw a yolk sack.  She asked if maybe my dates were off, that I looked more like 5 weeks or that the baby isn't growing as it should be. 
My first pregnancy ended in October with an early m/c and I had a period in December & one in January.  Now I seem to be 3 weeks off?? I'm lost.  I don't know what to think. 
Anyone experience something like this?