Well, it's a vicious cycle with the spasms. The initial pain comes from the fissure when it's traumatized. That pain is felt through the nerves below the fissure, around the muscle. The hurting nerves cause the muscle to contract and squeeze a bit on the fissure. That causes more pain from the fissure, thus more contraction on the muscle and so on. So there you have a vicious cycle that goes on and on. Eventually the muscle is so contracted that it completely closes the anus and it feels like there is no hole anymore, just raw burning flesh.
Then another factor is the pure muscle pain you get from simply having the anal muscle contracted for so long. After 8-14h of contraction it feels like I've been lifting weights with my asshole, even the next day, all day long.