Polyethylene Glycol 3350 Bowel discoloration?

I've been using Polythylene Glycol 3350 Oral Powder on and off for three weeks now and have noticed that my bowel movements have taken on a dark, tar like consistancy.  I was wondering if this is normal? 

I am anemic and my doctors are looking for the source - I am hoping it doesn't have anything to do with my digestive system as vitamin and iron deficeincies have been ruled out.

 Thank you for your assistance,

Sheryl Linton