Brown spotting for 3 days?

I have PCOS and am taking metformin. My period started on the 20th and was light. Very few to no cramps. However I stopped having red blood now the past 3 days have been nothing but brown discharge with some stringy brown things in it. I haven't had a period last this long since I was 15(now 20). My normal period will usually last about 5-7 days. But this one just keeps spotting and dragging it on longer. My period before that started April 20th and was very light and lasted 5 days. So I'm kind of concerned why I'm still brown spotting past my normal 5 day periods. Any ideas why?