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.
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.
Oh thank God. This is my third child and I was beginning to worry myself to death.
It can just be your last period ! I was having a period with my 1st at 5weeks