EDD when using OPKs?

Has anyone had their weeks/edd be off when they've used ovulation tests? I got my positive ovulation on January 20. Pretty sure I ovulated on the 21st which would make me 7 weeks tomorrow. I went for my first ultrasound yesterday and doctor saw the sac but no baby. Sac measured at 5 weeks 4 days and she did find a heartbeat finally at the end of 111bpm. There must be a baby if there is a heartbeat right? Could the ultrasound be wrong on the weeks? If the ultrasound is right then that would put my ovulation around the 29th and we hadnt even had sex around then (the last time was on January 23rd). Should I just trust the fact that there is a heartbeat and not stress about the weeks since it's still super early? Just wondering if anyone else has anything like this happen? Or just any insight.... thanks in advance ladies!