yellow discharge before AF?

Al
AF is due in 3-4 days and I noticed this morning I had some yellow discharge but it turned light brown. I don't have any itching or burning or anything associated with it. 
I'm not entirely worried bc I got tested last month for everything when I went to the RE and all negative. Just never had this before.
Not sure if it's a preg sign or hormone changes. Anyone ever experience this??