What causes brown discharge?

Sorry for TMI but I am 32 years old and had my tubal ligation 7 years ago during my c section. I have very regular periods, a couple days ago I noticed brown discharge, I am suppose to start in about 9 days and have never experienced something like this. I thought it was my cycle changing since I got off some meds then back on but it hasn’t turned into a period. I am gonna call my dr but just wanted some other opinions before I over react. Thank you!