Can weeks be different from lmp to what ultrasound shows? i should be 7 weeks and ultrasound show only a gestational sac at 5.3 wks.

Sono better than lmp: There are 2 possibilities. One is that you conceived later than you thought, perhaps due to late ovulation. The other is that the conception failed very early and has stopped growing. At 7 weeks there should be a distinct embryo and fetal heart visible on vaginal scanning. A repeat scan in 7-10 days will show if the 5.3 week sac is developing normally or not. Good luck.