I can understand why this is stressing you out, especially when it's been going on for so long. The good news is your doctor has checked multiple times and given the all-clear, which is reassuring even though they can't pinpoint the exact cause. Brown discharge is pretty common in pregnancy and usually not a sign of anything serious, but since you're also seeing bright red spotting now, it might be worth asking your PD specifically about the low placenta placement and whether that could be contributing, just so you have a clearer picture and can stop worrying so much. In the meantime, try to take it as a good sign that your scans have been fine each time. The anxiety is real, but you're being monitored, which is what matters.