Calculation based on last period can be inaccurate, especially if you ovulate late or if you have irregular cycle. Even if you have regular cycle, it wont always be accurate. Mine suppose to be 7 weeks if based on last period however when i went for scan i was only 5w, and same like you, only sac and small yolk, which is the very very small dot. (Now im 29 weeks) I too was super super paranoid and even cried but i read online its because its too early. Best to go for ultrasound when youre 8 weeks, cs by then can already see fetal pole or fetus itself, with a heartbeat. And yes doc will always say to come back in 2/3 weeks time for better ultrasound cs around this week they can hardly see anything or conclude anything. Doc also ask me if i had any bleeding cs she thought it might be fetus not growing or something. But dont worry mummy and dont overthink! Baby will grow well and you can start seeing their little heart flicker by 8 weeks! Start on folic acid everyday, and look out for any discharge or blood. All the best!!