Sophie I'm sure everythings fine - Didn't you have a scan recently and everything was ok?
Bleeds in pregnancy are very common and most of the time a reason can never be attributed to its cause. In healthy pregnancies many are put down to infection or hormonal changes, especially if you are around the time your period would be due if you weren't pregnant. I've had pink and brown bleeds with this baby and Ky between 8 and 9 weeks and none were down to anything sinister. I had a bleed with this one the day after sex, so put it down to that as I used to bleed sometimes for the same reason because I had cervical erosion.
I'm sure everything will be fine, but I do appreciate its hard not to worry!