Can still be implantation spotting at this point....could be from the cervix, which may or may not be from sex, it's pretty sensitive.....sorry though, I know spotting sucks!
Sounds like your u/s was great though, and they say brown spotting is fine, since it's old blood.....and I've had way more bright red bleeding than anyone I know and it's all turned out fine too. I had spotting (red) with DS during the first trimester, and that turned out fine, and you already know I've had more than a ton of bleeding with this pregnancy, and I'm 8 days away now.
Keeping everything crossed for you, I hope it keeps going great!