Unexplained bleeding 9 weeks 6 days

I went to the Sunday morning, woke up with a small amount of brown blood. I have been lightly spotting brown ever since. At the ER the did an ultrasound baby looked great, levels where they should be. Went back to my obgyn today and they basically said they reviewed everything and there is no reason for the bleeding so just keep an eye on it. I am freaking out.. has this happened to anyone who went on to had a healthy pregnancy