Pregnant while pregnant?!!! MISCARRIAGE QUESTION

So basically I think I was from 8-10 weeks pregnant and I’ve been miscarrying all weekend. I went to the hospital today where they did an internal scan and they confirmed the miscarriage was complete and nothing was left inside of me. BUT they said they’ve found a 4 week sac from a possible NEW pregnancy.. which I thought was basically impossible? I can’t find one thing online to do with this and I’m not getting more scans or checks for a few days. I haven’t got anymore information but that’s the worst thing they can tell someone that’s just had a miscarriage if they’re not sure..

Has anyone heard of anything like this before?