Why did Montresor feel sick at the end?

Montresor does not experience satisfaction with the perfect crime he has committed. Instead he says:

My heart grew sick; it was the dampness of the catacombs that made it so.

He has called to Fortunato several times without getting a response. He is now all alone in the Stygian catacombs. This is perhaps the first indication that he feels any guilt or pity. He does not appear to want to admit that he feels anything but hatred for his victim and satisfaction for his accomplishment. So he attributes the heart-sickness he feels to the dampness of the catacombs. But the reader might feel that this is nothing more than a rationalization. Montresor has been down in these catacombs for a long while and didn't feel any such sickness before. It seems appropriate that Montresor should say that his heart grew sick, since the reader would be experiencing a similar feeling at this point in the story.
The author, Edgar Allan Poe, has achieved the "single effect" he was aiming for. Now it seems like he wants to end his story as quickly as possible. The "heart-sickness" he attributes to his protagonist enables Poe to "wrap up" his story in a few words. Montresor says:

I hastened to make an end of my labour. 

We might think that Montresor just wants to get away from the ghastly and oppressive scene of his crime. In only three more sentences he finishes the wall-building, plasters the entire wall over with mortar on the outside, and replaces the rampart of bones he had previously torn down. Then in only one more sentence he leaps forward fifty years and informs the reader that his victim's body has never been discovered. Did Montresor's own abhorrence at his crime cause his heart to become sick, and did that sickness motivate him to finish the job as quickly as possible? Or did he truly not feel guilt or remorse? Ultimately, it's up to the reader to decide.