Measuring small @ 7 weeks

Hi. I went to ER last nite for what I thought was a UTI which apparently I don't have, but because I've had some spotting she did an ultrasound. She saw a sac and a yolk sac but that's it and said I'm 5-6 weeks. I know my LMP but unsure of O. Do you think it's possible I I'd late? I'm trying to stay positive. This happened in December & at 8 weeks they had me take the MC pill for a blighted ovum. I'm sad and scared. TIA