Call your doctor's office asap when they are open, just so that they know and because you are worried. I had a rediculously small amount of brownish discharge (I only noticed it when I went to wipe, not enough to show on my panties) at 6-7 weeks, it stopped and I had no further issues. It is just oxidized/old blood. If it becomes excessive, turns to red blood, or you have cramping you can call the doctor's office and you can get ahold of the on call doctor anytime day or night.
But I had an ultrasound at 8 weeks that showed that I am having twins and a small pocket of blood which sometimes happens upon implantation but your body generally reabsorbs it with no problem.4 weeks later it was nowhere to be found.
Normal. Worry if it becomes bright red.
Spotting can happen off and on during a pregnancy, and brown usually means old blood. I dont think you need to worry unless it changes to bright red in color