Prolonged spotting/brown discharge (no PICS!!)

Lauren
Starting at 6w5d, I've had brown spotting. I went to the doctor, saw our healthy baby and its heartbeat, and the doctor told me not to worry unless I develop severe cramping or the color turns bright red. And he couldn't visualize a reason for the bleeding (like a clot). He just attributed it to implantation. 
Well, I'm now 9w1d, and I have consistently had the brown spotting since then! It may stop for a day - but then it's back. I feel like a crazy woman calling my doctor, because they tell me the same thing every time...
But since I don't have another ultrasound until 12 weeks, I just find myself stressed out that something is wrong! 
Has anyone else gone through this? 
Thanks in advance!