I am not aware of that specific diet regime but if there are vegetables and fibre in it then I don't really see any reason why it should be so difficult to "go".
However, I recently had a very similar problem. I had changed a few things regarding supplements and couldn't understand why my bowel was so slow and difficult to move. Extremely firm and solid is how I can describe it. It was feeling bloated and uncomfortable.
Do you know what the answer was? I wasn't drinking enough fluid/water. As soon as I made sure that I drank more water, my bowel movement improved and was much softer and easier to move. I must of been de-hydrated. I suspect that a change in the supplements must of caused increased urination which dehydrated my body, drawing water from my bowel and hence constipating me.
Therefore, for a few days, make sure that you drink plenty of clean water or at least fluid of some kind.