Missed period? TMI Pic included.

Should've started period earlier this week. Had some very dark brown spotting that was very light and then nothing. Today I started cramping badly and had light spotting soon after that was dark and had a small glob of very sticky discharge with it. Why is this happening? I will say my breasts have been very sore which is not a usual pms symptom for me and I've had acid reflux and a cold. Any help would be great.