So sorry you're going through this!
Brown spotting is not always bad.
I've had several incidents of brown spotting, some pink spotting and worst of all, some red spotting. How terrifying...
However, I've had 3 scans and LO is perfect so far, even growing a bit big for its age! No sign of the cause. I know lots of other women have suffered this worrying symptom.
A couple things - pregnancy makes your cervix sensitive. So sex, pelvic exams and anything else can cause bleeding. Also, implantation bleeding can happen as late as 10 weeks (i guess it takes some time to work its way out?). Also, don't let brown blood scare you too much - brown blood = old blood.
Please see your doctor if you're worried!
Good luck and fingers crossed for you!