Can Ultrasound be Wrong?

I am irregular in my period, so I dont know when I ovulate or not. I had a blood preg test on 2/1 and it came out neg. On 2.15 i had a transvaginal u/s in which they said that all they saw was that the uterine lining is thick. On 2/18 I took a home preg test (actually 3) and all confirmed positive. On 2/13 I had a blood test again in which it came positive. I had an u/s on march 2 and they saw the gestational sac and said I was about 4 wks preg. I had an u/s yesterday and they told me I have blighted ovum. Could it be too soon? The dating on my pregnancy is not set in stone. I haven't had any bleeding, cramping or any termination signs. I have read many stories of people being told that and they waited and boom there was a baby? Any people with experience or knowledge please