Unusual menstrual cycle

I'm 36, was due to start my menstrual cycle yesterday but started a day early, kind of heavy at first and very dark brown. Yesterday was very light to almost nothing and still unusually dark brown. Slight cramping, but not like usual menstrual cramping. Took pregnancy test this morning and negative. I've tried Google with no real answers. Has this happened to anyone else?? Input greatly appreciated...Thanks!!