Why am I spotting?

Hey ladies. I have been TTC for over two years now. Today I am on Cycle Day 13, with a negative OPK test. I noticed I was spotting some, almost like the start of my period. What could this mean? I don’t have any cramps and have always been on point when my period starts. I have NEVER spotted in between cycles. I took a pregnancy test just because and it was negative.