What you are experiencing is not uncommon when doing a p&b colon cleanse. You may have loosened a a significant amount of mucoid plaque that is clogging the pipes. It will move through...but you can help it along and feel much better if you take an enema or colonic. Cut back on the p&b until the constipation subsides. Also, make sure that you are drinking plenty of water.
You might want to take a look at my post: Many Causes of Constipation...which you can find on the Bowel Cleanse Archive forum. It may give you some other ideas about what is going on and how to address it.