Brown spotting for 2 days

7 weeks along. 
Started having brown spotting yesterday morning...not heavy.  Went to ER last night and they still detect a heart beat, hcg at 64000, and closed uterus. I'm still spotting today.  They didn't seem worried about miscarraige. I'm scheduled to have another sonogram on Wednesday. 
Anyone else had this?  I wish I knew why it is happening.