A lot of women don't have u/s this early in pregnancy, so this may be a common condition that goes away on its own w/o a problem. Also, u/s is not always perfect -- numerous stories abound of babies being identified as the wrong sex; major or minor problems that either resolved themselves by the next visit or by the end of pregnancy, or perhaps weren't there to start with; vastly wrong differences in weight (u/s predicted 10 lb baby, so the mom had a C-section with a *surprise* 6 lb baby who then needed to be in the NICU because s/he was actually premature).
This is a huge reason why I am opposed to u/s in the early part of pregnancy -- more often than not, it gives mothers a reason to worry, for what is really no reason at all, and even if there is something going on, **there's nothing that can be done anyway**!!! Worry certainly doesn't help, and may even hurt -- yes, stress causes changes in your body, and does affect your baby. What can be done now, even if the heartbeat is abnormally high? Absolutely nothing (except perhaps you can work on stress reduction, take relaxation courses or something -- which is a good thing to do anyway). What can be done now if there is a blood clot in your uterus? Absolutely nothing. What is the point of you worrying about it? Absolutely nothing.
I know you're nervous and scared about the blood clot, but the doctor says it's no problem, so expect it to be no problem. Ultrasounds are not without risks -- the FDA says they should not be done except for medical necessity (although doctors do them even when they are not medically necessary all the time). There really have been no long-term studies about the safety of u/s, and particularly at the rates most women these days have u/s, nor with the higher-quality u/s. Doctors are looking into using u/s as a method of sterilizing men, but we're using u/s to look at developing babies. Doesn't sound like a good idea to me. We know that u/s does have effects on body tissues -- but what effect is it having on growing babies?