I have heard that about 30% of women spot during the first trimester... You will find that a lot of women on here had either spotting, or bleeding, and went on to have healthy babies... myself included :o)
I had spotting with my daughter and she is now 3 years old. My dr said the same thing, I had an u/s and everything was ok. It is worrisome but as zevasmom said, it's common in the first trimester.
Take care
I also had dark spotting with my first pregnancy, although a little later (6 weeks) and went on to have a perfectly healthy baby. I have heard if it is dark it is not something to worry about as much. Just a warning, not to give you too many details, but when I went to the restroom more dark blood came out. I remember this really freaked me out at the time, but everything was fine. I am just telling you this in case the same thing happens to you, so you won't worry too much! I have also read that implantation bleeding usually shows up around a week after your missed period. So, in your case, the timing is just right for the implantation bleeding. That is probably what you are experiencing, but I am not a doctor....
I am sure everything will be fine if it was just a tiny bit of brown spotting that you had. It is very common. Keep me posted tomorrow with your good news!!