First scan at week 7

Only a sac can be seen. Quite worrying, doc implied ectopic pregnancy is possible. Asked to go back in a week for a scan or do immediate blood test now and in next 48 h. Chose to wait for to scan again next week. Don’t know what to think 😢 wonder if using last period first date is accurate. No pain or bleeding. Maybe very light brown spotting seen 3 days ago?