13 weeks and brown discharge/spotting

Jessica
It has happened the past 2 days. My doctor isn't concerned about it but I still am. She listened to the baby's heartbeat and it was still great. I just don't know what this could be caused from???
No I don't have sex
No I haven't had any exams lately. 
Suggestions? 
I also have been drinking a lot of water since the appointment but it still happened yesterday once.