So confused…

I had a miscarriage about 4 months ago. I f found out I was pregnant again 3 weeks ago & my last period was October 9th. Well, I should be 8 weeks 5 days today. I went in for an ultrasound and they said my sac is measuring 5 weeks 6 days but the fetal pole wasn’t measuring up to that. They are concerned it might’ve stopped growing at 5 weeks. My stomachs been hurting a lot lately I keep having to poop. And i feel pregnant. Like I am nausous, bladder feels full all the time, out of breath, ect.. I go back next week to see if there’s a heartbeat but she didn’t sound so sure that this isn’t a missed miscarriage. Has anybody had this happen before and everything was fine you were just ended up getting pregnant a couple weeks after your last period?