Feeling discouraged

I just went for another ultrasound, I went on Thursday and the sac measured 5w2d and the fetal pole measured 5w5d. I went today and she said the sac was alittle bit bigger but not much change otherwise and she seemed concerned. She said maybe I'm not as far along as we think since we really don't know when I got pregnant. My question could it be that I'm really 5w6d as opposed to 6w2d and maybe the sac needed to grow to make room for the baby and that's why there's not much change? Idk, I go back Friday for another one. Someone please give me some advice or encouragement. This is my 4th pregnancy, I've had 3 miscarriages.