As per your last menstrual period, your gestational age
is around 10 weeks
but the ultrasound
is showing a sac of only 5 weeks
Also, please make sure that a TVS ( transvaginal scan ) was done.
On TVS, yolk sac
is definitely seen by 5 weeks, fetal pole
by 6 weeks and cardiac activity too by 6 - 7 weeks.
SO your ultrasound is definitely not normal, and suggestive of a failed pregnancy
However, if your periods were not regular, sometimes, there is a chance of delayed conception.
Please mention pattern of your menstrual periods.
Even then, the disparity between LMP and ultrasound is too large.
THe pregnancy appears non viable, and you should go for termination if a reliable TVS report also confirms the parameters you mentioned
Please consult your gynecologist at the earliest