Hi...
Unfortunately, this is a "normal" occurance when taking the iron tabs. Mine is that way as well, and while I was in hospital, I had the same sort of stool. I was so scared that after I had "done my business," I ran out to the midwives and asked why my stools looked like tar. She said it was normal. Yay.
It's not nice, is it?
Hope this helped!
--Amy