Bleeding at 8 weeks

Coryanne • Pregnant with baby #3!
I am just at 8 weeks and last night I started to bleed. It was a little bit bigger spot than a silver dollar in my underwear. It has since stopped as far as I can tell, but what would have caused that? Should I be worried? I already had an ultrasound and saw a heart beat and the doctor said everything looked good, but of course I am still worried!