13 weeks - TMI spotting

So I am 13w 3d and have a NT last week that came back with great results. Baby's HR was around 157 I believe and everything looked good on the ultrasound. Yesterday afternoon I found some dark brown spotting in my underwear (about the size of a dime) and some when I wiped from going to the bathroom. It stopped for a little while and then I had a little on the toilet tissue when I went to the bathroom last night and it has continued like that since then. I called the Dr on call and she said not to worry that brown blood is old blood, and only be concerned if its bright red. I've not really had any pain except for a little pain near where my ovaries are, which I have felt every so often since getting pregnant. I know the doc said not to worry but boy am I concerned. This is my first pregnancy and I'm so excited for my little nugget. Have any of you experienced this before?