8 days of spotting?

Sarah

WARNING TMI....Hi, so little background I've been ttc for about 2.5 years, I have had a progesterone test tell me that I do not ovulate -it was at a .8 if I remember correctly, but I haven't had any further testing. So I have very irregular periods, which are usually pretty heavy and last about 4 days. But this past period, there was no symptoms, no cramping (maybe some mild ones right at the end) but it was 8 solid days of very mild spotting. Does anyone know why the sudden change?