Okay could someone help me out please

So I had a miscarriage on October 30th and then when my period didnt turn up I tested and it was a very faint positive on the 8th December,

Had a scan on 19th December and saw a gestational sac and yolk sac that measured 5 weeks and 2 days, yesterday I had another scan that we saw a fetal pole and heartbeat but it measured 6 weeks and 2 days....

I dont have another scan till I'm 12 weeks but obviously I dont know when I ovulated,

So counting back using 6 weeks and 3 days (today) means that I roughly ovulated a week before I got my first positive test? Which cant be right, although if I use my first measurements then I ovulated about 13 days before I got a positive test?

Is it bad its measuring smaller? They didnt seem worried at all, said their measurements can have a 5 days margin because it's so small....