It can just be your last period ! I was having a period with my 1st at 5weeks
Oh thank God. This is my third child and I was beginning to worry myself to death.
It could be implantation bleeding.
It's usually not too heavy but in some people can actually be very much like a period. Cramping and all.
Sounds to me like it's normal, and since your doctor reassured you with an ultrasound you and your little one should be just fine.
it could be a subchorionic hemorrhage. not a big deal just one of the many causes of first trimester bleeding. most resolve I their own by the end of the first trimester.