Spotting on and off, normal?

I started spotting around 14 wks for a week, red blood in the evening and brown during the day. It stopped for 4 days then started again but red I the morning brown through out the day. It just stopped again 3 days ago and I'm 18 wks. I want to the ER at 16 wks and they didn't see anything. Has this happened to any of you? My Dr. Said not to worry unless it's more than 10 days in a row. It's hard for me not to stress, especially since I didn't spot during my last pregnancy. What could it possibly be?