I had the same happen with my first. First and second missed periods had some color come through, and I was cramping too. Scared the hell out of me, but the doctors assured me it was older blood that just hadn't passed. The cramping was normal too, they suspected it was more ligaments stretching than anything else. I spotted throughout my first pregnancy, randomly. The docs kept telling me to watch out for dark color, pink or brown wasn't typically bad unless it got heavy or lasted for more than 24 hours. But. If you're stressed out or worried about it, go see the doctor. They're more than happy to assuage any worries and help you out. Congrats and good luck!
Most likely your fine as many women after sex and or the beginning still have left over blood from last menstruals! I bled a normal menstrual and was pregnant for a month and a half! If your stil unsure speak with your dr. I know how the feeling of a new mom feels on my second baby's right now...
I was spotting around week 5. They did u/s and they saw the bleed but the baby was fine, goof heartbeat. Try not to worry. It happens pretty often. Google subchorionic uterine hemorrhage or read here http://www.**********.com/400_i-have-been-diagnosed-with-subchorionic-hemorrhage-is-there_2005602_334.bc