Brown blood is usually old blood. If it turns to bright red, then I'd be worried.
Brown blood is a indication of implantation bleeding. It normally starts around 6 weeks that can last up until 10+. Also it's considered old blood since it is brown meaning it was a slow process coming out. Implantation occurs due to the fetus implanting itself and the placenta to the lining of your uterus causing skin to Shedd. It's 100% normal and even normal to see pale pink. If it becomes red like a period or even dark red and seeing blood clots that is a sign that something more is happening and I would go to the er!
It is probably from you embryo implanting deeper into your uterus. Brown blood is old block, so try not to worry. Drink lots and lots of water to keep hydrated. 80ozs or more a day