M.W.
It could be implantation bleeding (when the fetus implants into the uterus - usually this is painless bleeding) - this is very common early in pregnancy. Consult your OB but meanwhile just take it easy - I'm sure it will pass.
Was the ultrasound internal? Sometimes that early in a pregnancy it's difficult to see anything on an external ultrasound (or sometimes an internal as well!)
I'm sure everything will be just fine!