Brown discharge, feeling worried :(

Rachel • 32 years old and just started TTC #2
Woke up this morning to a brown discharge when wiping (sorry if tmi). 
I just got my BFP yesterday and am at 4w3d. 
Don't really have any cramps but have slight back pain. I'm feeling worried that it might be a chemical preg/miscarriage. 
Has anyone had this/heard of this before?