Bleeding for over a week. 13 weeks I'm so scared

I've been bleeding for 9 days. It started out as period like blood. But never got into a really heavy flow. It was dark red for about 3 days. And since then it has fluctuated from pink spotting to red spotting. At least once or twice a day when I wipe there is blood present. Some days lighter than the other. It never really flows onto the pad. I've had 3 ultrasounds since, the last one being very detailed. And they all have come back fine. Both babies are moving with great heartbeats. They have no explanation as to why I'm bleeding this far because nothing is showing in the scan. The doctor said I'm too early to detect previa. And everything looks good. Is this normal? I'm finding it hard to believe. I've had no cramping. Just lower back ache and my thighs ache sometimes. Have anyone experienced this?