Hi there, I'm new here and just saw your post. One thing I thought of....if your pup is on pain meds maybe they are upsetting his stomach and causing overall irritation. This happened to my yorkie when he got neutered.
My dog was showing those same symptoms latinsavy. He had a small round protrusion sticking out of his butt and it bled off and on. The folks at bansfield said it was a hemorroid and too wipe his butt after number 2 then apply Prep H. It's shrinking fast and is scabbing over which is a good sign. I'm not sure if this is your deal so going back to the vet is your best bet!
Here's a pic of it a few days ago. http://i6.photobucket.com/albums/y205/child_n_light/IM000471.jpg