Hello everyone! My last period was on July 31st and the program shows …

Hello everyone! My last period was on July 31st and the program shows 6 weeks. But today I went for an ultrasound, it says that I'm 5 weeks pregnant. Last week I saw another ultrasound technician, then she said 5 weeks. How is this possible? I told her that I went a week ago and they put such a term. It says 5 weeks, there's no heartbeat but it develops well at its stage, but there's a threat and uterine tone. Girls, I'm thinking and worrying, am I having a miscarriage? I'm very afraid ?. If so, don't they see it?