usually after 5 weeks foetal hear beats are seen in sonography. since last week they didnot find heart beats they have rightly advised to come back next week ,as sometime if could not be appericiated ,due to any reason ,follow up sonography at 8 weeks will definetly determine the outcome.however other findings in sonography also help suspected miscarriage. mild stomach and back pain with nausea cannot definitely say anything. so its better to get another pelvic (vaginal) usg done after a week.