What Causes Pink To Brown Discharge Before Periods?

My period was due on March 18th, instead of my period I had four days of very light pink when I wiped. I have had brown when I wipe now, starting on the 26th. I have PCOS and my periods are sometimes a little late. I usually have one or two days of brown when I wipe, then I start a period. I'm a little concerned about what's going on with my cycles.