Check here - http://www.livestrong.com/article/221743-a-colonoscopy-post-procedure-pain/
A doctor should be notified ASAP if bleeding persists, or if abdominal pain, fever, chills, or difficulty urinating occur.
The two main complications of polyp removal are bleeding and colon perforation. These complications are rare, and bleeding can be easily stopped with cauterization - but it requires going back in to do that. Pain seems to be pretty rare as well - you need another opinion, probably soon as black stool indicates bleeding up in the colon and that needs to be stopped.
There should be no pain post polypectomy as far as I know. No one I know who's had it done reported pain.
Best of luck and get it checked out soon!
Welcome to the gastroenterology community! Did the doctor tell you what he think happened to cause this to happen to you? This is not a normal thing to happen after getting a polyp removed. I would recommend getting a second opinion ASAP.