Sometimes brown spotting in pregnancy is completely normal; I know I've seen lots of threads since I've been on here mentioning some kind of spotting and everything turning out okay.
If you're super worried, bleeding lots, starts turning red, get any pain etc you can always pop down to A&E if that's an option, or call 111 for advice? Hopefully it's just a bit of residual blood from everything smooshing around and stretching in there!
Melly above also has a point - have you irritated your cervix or had any injuries etc?