I Am 6 Weeks Pregnant, But I Am Spotting For

I am 6 weeks pregnant, but I am spotting for a day or two and them it stops. It's the third time now it happened to me during this pregnancy. I went by my doctor 4days ago when I got my second episode of spotting, I wasn't six weeks yet. She did an ultrasound (both tummy and vaginal), but found no heartbeat of the baby. She gave me some vitamins and told me it could be the beginning of a miscarriage and I will just pass it out. But she also said if it remains light or stops to come back in two weeks for another ultrasound. I am worried and just looking for a second opinion. Can u help me?