Ok So I Have Had Five Children All Close I.

Ok so I have had five children all close I. Fe. My oldest is 9 and my youngest is two. I am still nursing my two year old. My menstrual periods are normal but I seem to have some weird bleeding in between mixed with mucusy discharge. It's usually very brown, so it seems like old blood. I'm 36. Is this weird? I extremely active and eat well. Should I be concerned? Or is it my body just getting rid of old blood?