Could this pregnancy still be fine

So i have hcg draws every 48hrs and the first 3 more then doubled well the 4th bnb only went up by 100 and something. Dr said she has seent his happen and be fine. Im new to all this. Have any of u had experience with this? I have spotted some brown after a vaginal ultrasound after my 2nd hcg draw which i was to early to see a thing. I have no crampimg or anything but im really worried.