not really sure if its a miscarriage....?

So I had my first ultrasound December 29 & they said i was measuring 5 wks 6 days (this was at a clinic and the lady doing the ultrasound was a lady that worked in the NICU she was actually one who was with my son one night) but she was the one doing it and it was just a sack and a fetal pole? I guess that's what it's called. but I went for an ultrasound Tuesday at the regular doctor and she said baby was measuring 6 wks 4 d

ays. I haven't had any bleeding or spotting. I had a little back pain but more like a pulled muscle for a day. Not sure what to expect or think right now. Google is my best friend 😂 so I googled a 4 week ultrasound and it looks about how my first one looked. 🤦 just hoping for the best right now. but with everything going on with my mother-in-law && my son I honestly wouldn't be too upset. but has anyone had a missed miscarriage? or had this happen to them and it threw the due date all off? my last cycle started 11/4 so this is just all confusing. based off that o should be around 10 wks.

this is a pic of my first ultrasound.. they didn't give me a picture of my recent one.