I'm having similar worries. Right after getting my positive HPT almost 3 weeks ago I had brown spotting. It lasted for a few days and disappeared. I figured it might be implantation. Then on Sunday (two weeks after positive HPT) I started spotting again. It's not a lot, just when I use the restroom and it is either a very light brown or a very light pink. Sometimes there is a tiny tiny tiny streak of red, but I mean small. I have had blood drawn several times. First week after positive HPT it was 103. One week later 2000 and yesterday it was 15,000. I just have to wait and see. It is very terrifying I know but it sounds like it is pretty common. Hang in there and I will try to as well! I hate all this uncertainty in the first 12 weeks!
Brown blood means old blood, could take some time between the initial bleed and actually seeing it. As it has been over a week there is no harm in getting checked out to put your mind as ease. With my first two pregnancies i had implantation bleeding. I am now 7 weeks too and not had any up to now. Call your midwife/ doctor and they should put you at ease x