I went for an ultrasound Tuesday they did a vaginal one also. It was showing I was six weeks four days pregnant. There was no heartbeat. They want me back Tuesday to recheck. Is it common to find it after not seeing one?

Is it common to find a heartbeat at seven weeks and four days after not finding one at six weeks and four days?

It usually starts around six weeks, but depending on how they date you it could have not started or developed yet. It isn't something I would be too concerned about at this point.