Aiyoh, this happened to me too during my first pregnancy and it was quite worrying! Sometimes the dating can be off by a couple of weeks depending on your cycle length or ovulation timing, so it's actually quite common, but since you're only seeing the gestational sac at 5 weeks when you calculated 7 weeks, I think it's best to go back and see your doctor again in about a week or two for another scan to make sure everything is progressing nicely. They'll be able to track the growth properly and give you a clearer picture, so try not to stress too much in the meantime, okay?