Pregnant and confused

Hi all,

I'm in a very confusing/worrying position, and was wondering if anyone could share some experience?

I'm pregnant with my first, which came as I total surprise as I have not had a period for three years! I was beginning the process of going down the fertility treatment route, and was advised by my doc to take a 5 day course of Norethisterone (progesterone based pills) to bring on a period.

No period arrived. However, a couple of weeks after finishing the course my breast started to feel very tender. I got my hopes up thinking this may be premenstral symptom! still no period, but another two weeks down the line I started feeling very sick and threw up twice on morning. I thought I had picked up a bug, but my mum advised me to do a test and it came back positive! 

I based the conception based on the time i took the pills (stab in the dark, but the only thing that made sense) so expected to be about 5/6weeks. My partner and I went for an early scan yesterday (it was an internal vaginal scan) and I was shocked to be told that my gestitional sac is measuring at 11weeks but the embryo is only 6weeks! The yolk was there, but no HB.

nothing was explained, and the nurse told me (and I quote) to have 'pads and pain killers ready'. I'm booked for a scan next week to see if there has been any progression  

we were/are heartbroken and utterly shocked. But as the day went on, I started thinking about the 11 week date and things don't add up. My partner was travelling on business a lot 11 weeks ago, so during the proposed time of conception I am 99% sure we did not have sex. 

What is also strange is the the embryos 6weeks growth matches up to what I expected. 

I also did another preg test when we got home and it was positive  

I don't want to get my hopes up and think overally positivel, but has anyone else been told a similar thing? Can you have a surprising large GS while your baby is only small? Can this be the case with twins?

on the reverse, could I have been carrying a dying embryo for five weeks with no signs of miscarriage? I have had no spotting and discharge since my believed 6 weeks pregnancy. 

i know this is an essay, but if you have any thoughts, opinions or relateable experiences I would be so appreciative.

thanks for taking the time to read.

grace