I Finished My Period, Which Are Always Regular (like Clockwork),

I finished my period, which are always regular (like clockwork), then a week and a half after my period ended I started spotting. It went from spotting to bleeding and cramping and has lasted for a week now. I am 37 years old and I had my tube tied 6 years ago. I have also been feeling extremely fatigued, nauseated, and been having daily headaches. What could be the cause of this?