spotting two weeks before period brown

Emma
Ladies hello, so to start off no chance I'm pregnant, I'm also not on the pill.
I notice today that I had a medium amount of spotting in my underwear, brown blood. Kinda gross sorry. 
I am not due until 2 week, last period came super late, so it's odd that I am having a maybe period right now??
My Fertility window ended a few days ago. 
Any ideas or anyone go through this?!? 
Thanks!!