Measuring behind early

Basically I went to have an early dating ultrasound and it has left me feeling confused and certain I'm going to lose the baby. 
I estimate that my last period began either May 11 (spotting) or May 14 (heavy flow). That would have put me at either 6w 5d or 7w 1d. But the internal ultrasound measured me at 5w5d. Heart beat was 120bpm so strong and baby was where he or she is meant to be but just smaller than I expected. The sonographer wasn't concerned and said sometimes dating just doesn't make any sense. My Drs receptionist said moving dates is normal in early pregnancy. But I took a test at what then would have been 3w1d and that was positive instead of what I thought was 4w or 4w5d or something. I'm so confused please help me. I can't sleep or eat thinking I'm just going to lose this child, this so wanted baby.  
UPDATE
So just an update in case anyone reads this later asking the same sort of questions/going through the same thing. 
Today I went to the OB for the first time - I had been seeing my GP before this. I had levels (hcg) taken a few times between finding out my baby measured behind and today's appointment. They weren't doubling but increasing. 
Today's appointment basically began with my ob explaining that with my levels and my dating scan, we were most likely looking at the fact that we caught the baby when it started slowing in growth and most likely the scan today would not show a heart rate. I had sort of come to this conclusion myself and held my breath while he put the probe on and looked for baby - my husband was there holding my hand and I could tell he was feeling exactly the same. We had discussed it many times and he had constantly said that it would all be ok and baby would be fine but he admitted after today's appointment he had been thinking the whole 4 weeks wait that baby was most likely dead. 
Well after what seemed like an eternity we saw baby but unlike the other ultrasound which was done in a proper clinic with up to date tech etc I couldn't see any flickers of a heart beat. I'm holding my breath and can see my own heart basically beating away the probe from my stomach. Imagine my surprise when the obstetrician says 'there's the heart beat' and points to the screen and then measures baby at 10 weeks 5 days, then at 11 weeks 1 day when baby stretches out and waves his little hand at us. Oh my gosh, the relief. Ob basically said its remarkable and put my dates back to my last menstrual period date - taking off 8 days from the original move. So here I am, 11 weeks pregnant. Baby is ok. It's a miracle. Apparently some babies grow slower than others. He said there's many theories to explain it all but they are just theories. We can't know anything for sure. I'm having another scan next week to double check everything but ob said to be reassured after today. I cannot say how relieved I am. Just wanted to share in case someone else goes through something similar. As hard as it is, wait for a few weeks and then ask for another scan.