I had the exact same thing this pregnancy!!! I went for a scan at 7w 4 d and the Us showed a baby at barely 5w 2d. I to was thinking whats going on? Good news is that I went for a scan 3 weeks later at 8w 2d and I was actually 10w 5d

the baby was simply to small to see at the first scan,.. Good Luck,.. I hope you have another scan booked even if just for reassurance